> Stuff that used to be snappy is now sluggish and often entirely broken.

and as of when was GitHub known for its snappiness?

GitHub was very snappy in the early days. I remember how refreshing I found GitHub when it was new. I don't know when but sometime after 2020 it has just been going downhill.

The same could be said of Jira.

I could switch on the dev console in the Browser, and see which one loads the largest amount of Javascript, but I'd be disappointed with both.