I think Web apps have been around so long now people have forgotten how unresponsive things are vs old 2 tier stuff.
I tested the Jira cloud service with a new, blank account. Zero data, zero customisations, zero security rules. Empty.
Almost all basic operations took tens of seconds to run, even when run repeatedly to warm up any internal caches. Opening a new issue ticket form was especially bad, taking nearly a minute.
Other Atlassian excuses included: corporate web proxy servers (I have none), slow Internet (gigabit fibre), slow PCs (gaming laptop on "high performance" settings), browser security plugins (none), etc...
At that point, something mustve been wrong with your instance. I'd never call jira fast, but the new ticked dialog on a unconfigured instance opens within <10s (which is absolutely horrendous performance to be clear. Anything more then 200-500ms is.)
it's open within 1-2 sec (which is still bad performance, objectively speaking. It's an empty instance after all and already >1s )
It's still sluggish compared to a desktop app from the 1990s, but it's much faster than just a couple of years ago.