The comment I replied to said that trackers are slow because they are full of stuff, and when you try a new tracker it feels fast because of lack of stuff.
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.
"New issue trackers feel faster for the same reason switching browsers tends to feel faster—you're getting rid of all of the crap you piled up in the old one. Don't migrate your backlog, start with only a couple engineers in a new issue tracker, and suddenly, wow!, this new tool is so much better!"
I guess it can be read either way.