From github: https://github.com/mozilla-mobile/fenix/issues/18078.

This happens on Mali-T GPU device. Given that webrender will be enabled on Mali-T devices on firefox 87, I wonder if some fix will land until then, or will we get a performance regression from opengl.

Steps to reproduce

1. Open dobreprogramy.pl
2. Try to scroll up and down

Expected behavior

Scrolling should be smooth.

Actual behavior

Scrolling is not smooth, it looks as if website is moving by smaller/bigger distance on each iteration of scroll. So when you look at website as it moves, you see it jumping up/down a little (it doesn't mean I observe website moving in opposite direction as scroll direction, just that distance it moves every frame is not "consistent").

I tried to capture on video, but honestly it's not easily visible. The difference is "huge" when looking at device "in person".
https://www.dropbox.com/s/skgaczwpqoygfl3/2021_02_19_12_17_09.mp4?dl=0

When webrender is disabled in about:config, after browser restart, scrolling on this site is butter smooth.

Here is a performance profile captured with webrender on. In this run, website is already loaded when performance profiler is started.
https://share.firefox.dev/37reWki

Device information

• Android device: Galaxy J7 2017 on android 8.1
• Fenix version: Nightly 210218 19:31 (Build #2015794073)

Change performed by the Move to Bugzilla add-on.

See the GitHub issue for context: there's a long discussion there.

To summarize the github thread, the main issue is that the device is a Mali-T830 MP1 which means it only has 1 shader core. This leads us to be fragment shader bound, as can be seen by the profiles being dominated by eglSwapBuffers.