What I really hate is not Jira itself, it's the company behind it and its engineering practices. Their mediocrity goes beyond Jira.
They've had server-side-rendered features that while slow, contained the slowness to the backend and bundled it into one single slow request - once you waited for that, you had a fully-working page in your browser. Since it was all on the backend, your browser remained responsive while waiting.
Nowadays however there's no longer a single operation to wait for - they've split their pile of shit and offloaded it to your browser. You have to wait for the backend which is slow, but then you also have to wait for megabytes of JS to load & execute in your browser, all while the page moves around and your CPU is pegged at 100%. And God help you if you click on something by accident, as "undoing" that means waiting for another load. To add insult to injury, everything is clickable and triggers some actions, even elements that represent content and that you wouldn't normally expect to be clickable (nor would want to - let's say you wanted to copy the text).
Recently they've added another annoyance - in Bitbucket they've done some changes with regards to diff rendering and obviously have to let you know. They do so via a persistent floating popover in the bottom-left of the screen that you have to manually dismiss and the dismissal status seems to be contained to the browser - if you switch browsers or use private mode you have to deal with it continuously.
My experience with Atlassian products has actually deteriorated despite CPU speeds, browser performance & network connections improving. I used to have a mostly positive opinion of the company 5 years ago but that's no longer the case.