Is Your Scroll-Based Animation Killing Performance?

Still attaching scroll listeners to trigger animations or load content?
Here’s the practical, reliable way to ship smooth UIs at scale:
Intersection Observer

WHY?

The Hidden Cost of Scroll Listeners
At first glance, scroll listeners seem harmless.

You scroll → JavaScript runs → something animates.

The problem is how often that code runs.

Scroll Events Fire A Lot

When a user scrolls, the browser can fire the scroll event up to 60 times per second (once every frame).
That means your code is running:

• While the browser is drawing pixels
• While it’s calculating layouts
• While it’s handling user input

Your scroll handler is now competing with the browser’s most time‑critical work.

Small Checks Can Trigger Big Work

element.getBoundingClientRect();

But under the hood, it may force the browser to:

• Recalculate styles
• Recompute layout
• Flush the rendering pipeline

The Cost Grows with Every Element

• If you check one element, it’s manageable.
• If you check 10 elements, that work runs 10 times.
• If you check 100 elements, the browser struggles.
• This is why scroll‑based logic often feels fine at first……and suddenly becomes janky as the page grows.

Why Throttling Isn’t Enough:

“I’ll just throttle my scroll handler.”

Throttling helps reduce CPU usage, but it doesn’t fix everything:

• Animations can fire late
• Elements appear after they’re visible
• Fast scrolling causes missed triggers
• Touch scrolling still feels laggy

In other words: fewer explosions, same fire.

A Quick Performance Smell Test
Open Chrome DevTools → Performance, record a scroll, and look for:

• Long “Event (scroll)” blocks
• Frequent Layout or Recalculate Style steps

If you see them repeating during scroll, your page is doing too much work.

Key Takeaway

• Scroll listeners aren’t bad because they’re wrong.
• They’re bad because they make you do the browser’s job—over and over again.
• That’s exactly the problem Intersection Observer was designed to solve.