We put a lot of thought in our most recent release to improve the UI/UX, so some of your problems with that may have been resolved since you last used GitLab.
As for performance, we've started treating speed as a priority, and we've been tracking progress in various issues:
Yes! I've been seeing a lot of great improvements being added to GitLab recently. I'm intrigued to see what the future holds, and could definitely see myself switching in the near future.
With regard to UI/UX improvements, I'll have to check it out and see how much it's changed since last time :-)
If you have specific issues with the UI/UX or find any bugs, feel free to open an issue and mention me (@connorshea) :)
Personally the performance issues have been super annoying for me when using GitLab.com, so I've been doing what I can removing unnecessary assets and things, making the page size smaller.
One huge improvement recently was speeding up the Files view, I used to actually avoid it because it was way too slow, really bad experience.
We all use GitLab.com every day to develop GitLab, so we're very much aware of the problems!
With regard to UI/UX improvements, I'll have to check it out and see how much it's changed since last time :-)