How to Profile a Real-Time UI

The dashboard came up in under a second, and every loading number was green. None of that was the problem. Once the feed went live, with a vol-surface fit recomputing on every tick, the chart began to stutter and lag.

That is a different kind of problem, and you find it a different way. A load audit runs once and hands you a score; this you profile, by driving the UI under its real workload and reading the trace. The hard part is knowing what to read. By the end of this post you will know, for each way a real-time UI comes apart under load, exactly where to look in DevTools and which number matters.

Load is not run-time. Loading is the cost of coming up; run-time is the cost of staying responsive while the data keeps arriving.

Performance isn't one speed

Inside the run-time regime, "is it fast?" stops having one answer. A live UI is governed by three independent rates that keep competing long after load completes. They are a producer, a transformation, and a presentation; in a browser UI they show up as input, compute, and display, the last being the rate at which the screen itself can present new frames, fixed by the hardware at 60 to 120 Hz. The one rate you set is none of these: it is the cadence at which you commit to the screen, the knob you tune between them.

Performance is governed by competing rates, not a single speed. The diagnostic unit is not time; it is the relationship between three rates.

Run-time failures are mismatches between competing rates, not a single measure of speed. Compute numbers read off a 5.07 s Performance trace of the demo under a 500 Hz feed, 249 fits; a fast feed genuinely outruns the fit.

Used this way, the model is a procedure, not a taxonomy. Performance debugging is relationship debugging: you are not measuring one speed, you are finding the two rates whose relationship has broken down. The procedure is the same every time.

1. Identify the input rate.
2. Identify the compute rate.
3. Identify the display rate.
4. Find the pair that is out of step.
5. Measure that mismatch.
6. Narrow the class of likely fixes.

The rest of this post runs that procedure once per mismatch: every diagnosis below is these six steps applied, with the DevTools surface and the number for each.

You profile it, you don't audit it

A page-load audit works because loading is predictable: every user loads the same bytes the same way, so one automated pass characterises it. Run-time is the opposite. The expensive work is triggered by what the user and the feed do, it is specific to your workload, and it sits on no single path. You do not audit it; you drive the actual scenario, open the chain, fire the vol shock, drag the slider, and record a Performance trace while it runs. That is closer to end-to-end testing than to a one-shot audit.

This is the honest place for Core Web Vitals. They are very good at the regime they were built for, the pay-per-load web where a faster load lifts revenue, and they stop here by design. Even Interaction to Next Paint (INP), the closest of them, times a discrete interaction; a feed that drives a chart on its own fires none. There is no run-time metric in that toolkit because a workload-specific cost generally has to be profiled under representative load, not captured by a single generic pass.

The load audit on the demo: a perfect Performance score, and silent about run-time. LCP 0.8 s, TBT 0, CLS 0.001. True, and not the question this post asks.

Read the three rates directly from the trace:

• Input: instrument it, or take the feed's tick rate (a performance.mark per event). It is a range, not a number: read it during a vol event, not a calm market, or you measure the easy case.
• Compute: the task's duration on the Main or Worker track, and it is two numbers, not one: the median governs whether you keep up with the feed, the p99 governs the worst frame. Both are ranges too. The same fit runs faster on your dev box than on a trader's locked-down laptop or a VDI session, so a local reading is an optimistic floor, not the number every desk sees.
• Display: the screen's refresh, 60 or 120 Hz, the ceiling your commits live under. Read your own commit and frame cadence against it from the Frames track and React's Profiler; that cadence is the knob, healthy when you throttle it well below the ceiling, broken when the feed drives it.

Once you can see all three, the failures are the gaps between them.

I will show each gap, and beside it what correct looks like, from demo.oracaus.dev, which fits a fifty-expiry surface to a streaming chain and stays smooth doing it. Its trace under the worst case is the reference for healthy: a near-idle main thread with the feed running hot.

The same UI profiled under a 500 Hz feed with a vol shock running: the main thread is busy about a quarter of the 5.07 s window, idle the rest, and the Frames track is clean. This is the reference for healthy.

The method on one screen

Here is the whole method at a glance. The rest of the post is one row at a time, with the DevTools steps and the healthy trace for each.

You see	Rates	Look here	Measure	Likely fix
renders far exceed paints; work thrown away	input > display	React Profiler commits; Main track	commits/sec vs frames painted	subscribe once, throttle commits below the display
data lagging, queue growing	input > compute	Worker/Main track; User Timing; queue depth	compute median vs inter-input interval	coalesce to the latest input
output doesn't match the input shown (correctness)	the tear, not a rate pair	on screen; coherence check	does the output's input match the input on screen (timing is only the precondition)	commit input and output as one snapshot
stutter, dropped frames	compute per frame > display budget	Frames track; Main flame; Performance Monitor	p99 frame time, dropped frames, layouts/sec	move compute off-thread; compositor, cut redraw, cut allocation

Re-rendering faster than you paint

Symptom. The component re-renders on every input event, far more often than the screen repaints. Most of that work is thrown away before anyone sees it. It is worse than waste: those renders run on the main thread, so they help starve the next frame, feeding the dropped-frame failure below.

Renders fire at the feed rate; only the three that land on a paint survive. The rest is thrown away.

Look here. Open React DevTools, switch to the Profiler, and record while the feed runs. If it reports that profiling is not supported, the build stripped the instrumentation: profile a development or profiling build instead (for React, the react-dom/profiling entry, which stays minified and production-representative). A naive component commits on every update, a dense picket fence of bars in the commit timeline; in the Performance panel, the same work packs the Main track between paints.

Measure. Commits per second against frames actually painted. If commits run at the feed rate while the screen paints at 60 or fewer, the difference is pure waste.

Healthy. Commits track a cadence you choose, not the feed's, and that cadence sits well below the display's 60 to 120 Hz ceiling. Profiled against a 500 Hz feed, the demo commits at a deliberate 5 Hz throttle, not 500, each commit a median 0.1 ms of React work.

Likely fix. Stop letting the feed set your commit rate: subscribe to the feed once and let the UI commit at a cadence you set, rather than threading every tick through props.

Compute falling behind the feed

Symptom. Results lag the feed, and under load the lag grows. What is on screen is several ticks old, and getting older.

Each fit outlasts several ticks, so its result is stale on arrival; without coalescing, the lag grows.

Look here. Open the Performance panel, record under load, and find the compute task on the Main track, or the Worker track if you moved it off-thread; read its duration. Bracket it with User Timing, a performance.mark at the input and a performance.measure at the result, to put an input-to-result latency on the Timings track. If you queue inputs, instrument the queue depth: a growing queue is the tell.

Measure. Compute median against the inter-input interval, not the p99. The backlog grows without bound exactly when the typical fit outlasts the gap between ticks: that is a utilisation fact, governed by the median (or mean), not the tail. You can sit with a p99 above the interval and a median below it, draining the queue on every calm stretch, which is bounded bursty lag, a different failure. Once the median crosses the interval, you cannot keep up tick-for-tick, so either the queue grows without bound or you coalesce.

Healthy. Input-to-result latency stays bounded and queue depth does not trend upward, because each fit runs against the latest chain with the intermediate ticks coalesced away, not because back-to-back execution keeps up. At a 36 ms median against a 2 ms interval, processing every tick in order diverges without bound; that is the broken case.

Full-surface fits on the worker thread, each a median 36 ms (about 57 ms at p99). The gap stays bounded because each fit runs against the latest chain, intermediate ticks coalesced away, not because the thread keeps up tick-for-tick. Off the main thread, so a fit that outlasts a frame never blocks one.

Likely fix. Coalesce: absorb the intermediate inputs and compute against the latest. This is backpressure's lossy path, keep the newest and drop the rest, so you stay one result behind instead of falling endlessly further back.

The answer that no longer matches the input

You reach this one by fixing the others, and it is the residual they leave behind. Coalesce the backlog away, with no async involved at all, and the one result you do show was still computed during a fit the input moved underneath, so it is paired with an input it never matched. Moving heavy work off the main thread, the fix for the budget failure, only widens that window; it does not create the bug. And you cannot tune your way out: input rate climbs with volatility and compute time climbs on slower hardware, so on a busy feed or a slow machine the compute loses the race, however fast it is on your own box. The answer has to be structural, not a faster fit.

Symptom. The output on screen is paired with an input it was never computed from, a frame that was never true. In the demo, the fitted curve pulls off the quotes it was built from.

Look here. Not in the flame chart. You see it on screen, the fitted curve pulling off the quotes it was fit to, or you catch it with a coherence metric. The timing precondition, though, is visible: the compute taking longer than the gap between inputs.

Measure. A coherence check: does the input the output was computed from match the input on screen. That check, and not any timing statistic, is what makes this its own failure. The timing condition, compute outlasting the gap between inputs, is only the precondition, and it is shared with the backlog failure above; it is not the measure here.

Healthy. The output on screen always matches the input on screen: every pair is committed from the one snapshot it shares.

Likely fix. Commit the input and its output together as one snapshot. This is the one failure the profiler cannot show you; I took it apart in full in its own post.

A frame that misses its budget

Symptom. The visible jank: stutter, dropped frames, a frame rate that sags and spikes. The work for a frame overran the display's budget, 16.7 ms at 60 Hz or 8.3 ms at 120, and the frame missed its slot.

Look here. Open the Performance panel and record under load; the Frames track flags long and dropped frames, and clicking one opens the Main track flame chart showing where the time went, scripting, style recalculation, layout, paint. For a live read, open the Performance Monitor (press Cmd+Shift+P, or Ctrl+Shift+P on Windows, and run "Show Performance Monitor"), then enable CPU usage, Layouts per second, Style recalculations per second, and JS heap size; it plots them live as the feed runs, which a single trace cannot. The Rendering tab's Frame Rendering Stats overlays live frame rate on the page itself.

Measure. Read the worst frames, your p99, and the dropped-frame count straight from the Frames track; the long tasks (over 50 ms, marked with a red corner) on the Main track; and layouts-per-second live in the Performance Monitor. The mean frame time will look fine; it is the worst frames the user feels.

Healthy. Frames stay inside budget, the main thread runs no long tasks and sits idle between paints, and the redraw cost stays flat rather than climbing.

Under the hot feed the monitor holds steady: the JS heap sawtooths between roughly 50 and 95 MB without trending up, DOM nodes flat near 3,100, CPU around a quarter. Layouts and style recalculations hold in the single digits, peaking near 10 a second, never thrashing or climbing. No memory leak, no node leak.

Likely fix. It depends on which part of the frame is heavy. If it is scripting, a synchronous computation blocking the main thread, move it off-thread, usually into a worker; that is the compute failure above, surfacing as jank. If it is the rendering itself: move the animation to the compositor (covered here), cut the redraw work or drop to canvas, or kill the allocation the garbage collector is paying for.

Where this comes from

I did not assemble this from blog posts; I learned to read these traces on trading desks, in FX, rates and derivatives, where a frame that lies is a mispriced quote. The pieces are old and not mine alone: Nolan Lawson on main-thread cost, the reactive-streams world on backpressure, the React team on concurrent rendering, the game world on frame pacing. What I am adding is one repeatable procedure for reading them: find the relationship that is breaking down.

Try it on your own UI

You can put this to work in a few minutes, without any of my code. Take a real-time UI of your own, drive its worst scenario, and record a Performance trace while it runs, not while it loads.

Run-time performance is governed by competing rates, and a janky real-time UI is a mismatch between two of them. It answers a different class of question from your loading metrics, not a replacement for them.

Find the input. Find the compute. Find the display. Find the relationship that is breaking down. That tells you where to look next.