You're right, i'm sensitive to load times and exaggerated by approximately 4x.
I just (loosely) timed it, it took ~10-12s to open a backlog selected issue card _with cache_ from refresh. I used that example because it highlights the list i gave before.
1. The page loads, a bit slow in general.
2. The dom has loaded, so now backlog issues are loading.
3. The backlog issues are loaded, so now the selected component opens.
4. The selected issue component starts loading data.
5. Your data is now finally visible.
The ~10-12s is loosely evenly spread through the entire steps 1-5. This is on Jira Cloud, no "toaster NAS" unless you want to blame Jira Cloud for running a toaster, in which case i'd agree.
The problem in my mind isn't the servers. Opening the network tab, you see requests responding a bit slow, maybe 50-500ms, but not _terrible_. I'd like to see all requests below 300ms personally.
The problem is the UI design. Everything executes dynamically and sequentially. The URL indicates exactly what sort of page i want to see, but nothing is loaded until the JS loads, renders, makes a request for whatever data that individual component needs. Any sub-components to this then get rinse and repeat once they are actually loaded.
When you stack components on components on components that all need to sequentially load data a 1-2s load time starts to stack up, fast. And best of all, network requests are slowed down by how fast your DOM renders? Ugh.