Multiple seconds of lag when viewing/scrolling in specific Matrix room

Description

Scrolling in the #keepassxc:matrix.org room causes a large amount of lag, during which all inputs are ignored and at least 1 CPU core is pegged.

Steps to reproduce

• Join the room above with Element
• That should already reproduce it. If not, scroll upwards a bit.

Logs being sent: yes

Version information

• Platform: web

For the web app:

• Browser: Firefox 88.0.1
• OS: Arch Linux
• URL: app.element.io

[jonas-schievink]

The multi-second spikes seem to be gone now, although my flamegraph looks basically the same as the one you posted in https://github.com/vector-im/element-web/issues/17537#issuecomment-853475149.

I’m now testing on a different room that has even worse performance issues, #glasgow:libera.chat, and switching to that room causes the spinner to flicker in and out of existence a lot, while also spending a lot of time in _getEventTiles, and causing a lot of lag while it’s loading (giving maybe 1-2 frames per second on my machine until it is done loading).

Thanks for looking into this!

[robintown]

It looks like a large amount of that time is taken up by SettingsStore.getValue calls within shouldHideEvent. I might try replacing those with cached values from watchSetting to see if that could be a quick performance win.