What angers me about Jira isn’t the slowness of the backend - that I can wait. It’s the slowness of the frontend that’s the problem, which is a combination of a JS-heavy front end as well as its hard dependency on the backend, where as basic HTML forms & page navigations typically leave the previous page interactive until the response from the backend arrives so you can still look at the existing page as well as allowing you to cancel a page load and still keep the currently loaded page.
I opened up the api recently to discover that every story I edited had close to 100 custom fields on them, most duplicative (presumably) and hidden.