Not sure what happened to you - but speaking anecdotally, but from someone who logs into Jira Cloud every day, and spends 20-30 minutes interacting with the interfaces over 8-10 hours a day, and has done so for the last 3 years - it's had a handful of outages, nothing that really rose to the level of being memorable though (and less than any internal issue tracker I've worked with) - and the performance is, fine? I mean, it's not <50ms twitch fast, so, I guess I'm losing a minute or do wall-clock time + whatever larger blocks of time that occur when I get distracted waiting for a ticket to come up - but it isn't at all what I would call "unusably slow".
In our case though, we only have about 50 engineers and a few hundred thousand issues. Jira has performance issues in the 500 engineer / 100mm+ issue space - or it used to, perhaps they've resolved those issues by now.