re: Tell me a bug story VIEW POST


Recently we had a performance issue on a web app I was working on. We showed a series of questions where one question's answer may lead to a few new ones showing up.

The issue was that after answering the first question, answering the second question took 2 MINUTES to render the next set of questions. The UI was blocked the whole time.

I started to dig in. Our app let users manage 50 different products at once so this issue only started to manifest when you managed a ton of products. I also counted that the second question triggered 13 additional questions to show up.

When I started to log the number of Redux actions being dispatched, I was amazed to see we were dispatching (and re-rendering) 6000+ actions which is what was causing the slowdown. Each action dispatch was about 20ms (x 6000 =~ 2mins).

The questions had a graph structure where questions could relate to one another. Turns out we were following this graph structure and dispatching actions even if no actual data was changing, so I updated the logic to compare previous values; that cut it down to about 650 actions (50 products x 13 questions) which is what it took to make the new questions visible.

This reduced the time from 2 minutes to 25 seconds. Because the rest of the actions were technically needed to change state, I introduced the redux-batched-actions package to batch all those actions and dispatch one single action. Doing so reduced the time down to about 2 seconds. Much better!

Eventually, what I discovered was adding significant time was the JS ... spread operator. What! Turns out because we needed to support IE11, the spread operator was being polyfilled and this implementation was slow as heck. We switched some critical code to using assign instead and it reduced one functions total execution time from 1s to 12ms.

Overall, I got it down from 2 minutes to <100ms by doing these optimizations and simplifying the complexity of some functions to faster O(1) and O(N) implementations.

What a doozy! That took a good week to work through but the improvements were to core code in the app so the entire app benefited from it!


Wow! That is quite the performance improvement. If only redux warned if that many actions were dispatched, it feels to me like 6000 actions for one request is unlikely to ever be on purpose!

Nice job tracking it all down though. 2 minutes to under 100ms is amazing!

code of conduct - report abuse