As for performance, we've started treating speed as a priority, and we've been tracking progress in various issues:
https://gitlab.com/gitlab-org/gitlab-ce/issues/19273#note_12...
https://gitlab.com/gitlab-com/operations/issues/42
https://gitlab.com/gitlab-com/infrastructure/issues/59
We'll keep improving over the next few months, lots of work still to do.
With regard to UI/UX improvements, I'll have to check it out and see how much it's changed since last time :-)
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!