We do a lot of work at Clubhouse to keep things fast.
We have alerts set up that fire if the p50 or p95 of certain actions spike and we treat it as a bug if things are headed in the wrong direction.
We still see a lot of random cases of things being slower though (shakes fist at random browser plugin upgrades).
It's a hard problems, but if you don't design (and monitor) for this from the beginning things are going to slow down over time as you scale to hundreds of thousands of users.
Jira is so slow I installed plugins in both the devtools I use a lot to view tickets without having to use the web interface, it's just painful when you have to view a lot of tickets quickly.
Sluggish software can be more than just a data bloat issue. It can also be caused feature bloat, or software architecture that scales poorly.
Anecdotally, I recently worked on a project using a fresh Jira Cloud account. It was still slow.