Just try this out on linux (maybe works on other OSs): open a system manager with a real time CPU usage chart. Open the Slack desktop app (or a session in Chrome, it's the same). Open a channel (#random should work). Scroll up. Watch usage of all your CPU cores skyrocket. Stop scrolling. CPU usage goes back down.

They to hog all of my cores just to scroll up a list of messages.

BTW, this doesn't happen in Firefox: scrolling up doesn't cause CPU usage to go up measurably