slowness, I can accept, but the extremely bad UX, byzantine inconsistency, some parts are SPA some are long forgotten wizards with useless and completely idiotic next-next-finish steps, some functions are so close to useful, but ultimately fall short because even though Jira can do anything it can't simply graph a fucking JQL result set over time, it simply doesn't have anything useful to report by default. Epics and subtasks and story points seems useful, sure, we picked it because it has the best scrum support, but there's no aggregation of points from subtasks to tasks. Oh, yes you can DIY with ducktape-and-hate using the new automation functionality, but it'll have the same bad UX of course, because it's not integrated, because it's Jira. It's hell. It's bad. It's ridiculous.

It's unfathomably tragicomic how the alternative offerings are also UX nightmares. PM tools are cursed, it seems.

i'm not really sure what your metric is here, or if you're talking about their on-prem version. but the jira hosted by atlassian seems like it runs at about the same speed as most websites. i'm not going to say it's blazing fast, but it's acceptable.