It's hard to name a single action I can take in JIRA which does not feel unacceptably slow. However, these are the actions that cause the most issues for me due to being used most often (JIRA datacenter, MBP 2019 with i7 + 32gb ram):

1. Viewing a board. This can take 10+ seconds to load.

2. Dragging an issue from one column to another. This greys out the board, rendering it unreadable and unusable for 5-ish seconds.

3. Editing a field. I get a little spinner before it applies for 2-3s for even the simplest edits like adding a label.

4. Interacting with any board that crosses multiple projects. A single project board is bad enough, as in point 1, but we have a 5 project board that takes 20+ seconds.

Actually, I found an action that's pretty ok: Search results are fast, even if clicking into any of them is not. I'm not sure why rendering a board is so different performance wise.