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!