The functionality of JIRA is great. It's the UI that feels slow and clunky.

The functionality of Jira is great because it's basically turing complete at this point. A dedicated enough force of middle managers can warp it however they want to.

In the worst case sure. It also lets a small team start easily and modify it as they grow.

In many ways it's like Excel. Everyone says making an Excel killer should be easy because people only use 10 features. Problem is, everyone is using a different 10 features.

As another comment rightly pointed out, Atlassian has some recession "insurance," because JIRA tends to be bought by big, slow companies. Big, slow companies implement JIRA with the same agility and cleverness as they do their AD implementation, which is to say: it tends to be a nightmare of permission complexity caused by internal political machinations. That winds up making the UI slow and clunky, and driving the end users crazy.

Mainly it's slow and the UX makes everything painful.

I'm using Jira, too, and I don't think its slow. It actually works quite well and has loading animations for longer tasks, so it never feels laggy. Sure, it's not vi-levels of navigation speed, but it never bothered me. The UX works for me, too (although it took me a bit to learn it, to be fair).