I used to consult on JIRA a while back.
These complaints seem like they have a couple of potential causes.
One very common load time issue is downloading too much data at once, especially on boards. I understand some work has been done to make this better, but you may get some relief by creating new boards with very limited number of issues on it - you want the board's primary filter to be specific (eg a board just for stuff assigned to you, rather than for everything asssigned to your team).
The weird subviews tend to be configurable, and Atlassian has been moving more and more towards defaults (and locking down those defaults) for these configurations that push a specific persona/way of working. I'm sure they have data that supports those choices, and there is strong selection bias going on here, but almost all the consulting I did was trying to figure out how to work around those defaults (the simple ones you can just change!)
Probably, a lot of those issues could be fixed by your administrator - you may even have permission to fix them yourself, though that process can still be quite cumbersome and labyrinthine.
So many complaints about JIRA come down to complaints about how it's configured. Atlassian knows this, and I think they are trying to make it better, but it's a hard problem. I enjoy the endless customisation available as an admin but it takes time and effort to understand what's possible, more time and effort to design those changes, and the most time and effort to make those changes match what the teams need. It's a hard problem to fix.
Sometimes people, frustrated by this big bohemoth, will pick an opinionated tool that matches their way of working. This works great for as long as that tool keeps focused and the needs of the team don't grow.