Last time I tried switching to it it was slow as molasses on the command line. Easily several times slower than GitHub. So I went back to GitHub. I still have a GitLab account, but I only keep my dotfiles there.
I keep hearing they've improved performance, and maybe they did, but that's also what I heard last time I tried it, so now I'm kind of reluctant to give it another try. Unlike before, now it doesn't just have to be "as fast", for me to switch to it, it has to be noticeably faster. Feature-wise it's easily on par already, but performance is also an important feature, and I don't think codebase being primarily in Ruby inspires a lot of confidence, especially if scale is growing. You're basically running it at 1/10-1/30th the speed it could run if a proper language were used.