I think you might be misinterpreting him. When I read that, "full of stuff" meant old tickets, process that you had to go to, etc. Slow =/= app performance but general cruft.

I don't think he literally meant that the app's performance was slowed down by the volume of tickets. I don't think Jira's performance gets much worse even with 10,000s of tickets in it.

The baseline performance of the web app is just baseline bad though.