I agree. I couldn't stand the slowness of Gitlab's UI. I originally came to Gitlab because Bitbucket was lagging on several features that I wanted, and had a poor UI. Gitlab's UI seemed more inviting as it was similar to Github. Mostly though, I chose Gitlab for the free private repositories.

The Gitlab UI is /fine/, but the speed is what gets me. On github, even if I have thousands of commits, the UI is instantaneous. If I click on something, the load time is less than a second for me. So I switched to Github and paid for the private repositories. I absolutely didn't mind paying for this since Github is so fast for me and seems to be adding pretty cool new features (code review enhancements on PR, for example).

I still have one of my repositories on GitLab and it's still slow when I do things like browse commits, view source files, etc.

It's not just Gitlab though. Bitbucket is pretty slow for me as well, though not as slow as Gitlab. I would guess that Github's caching algorithms are much better than either of those two to really make pages seem snappy.