I dont think that is true, consider Fuzzy, Hey, Basecamp, Shopify all seems to be doing fine. Shopify actually got impressively fast in the recent two years I have no idea why. Github used to be fast as well.

So while Go by default will always be faster then Ruby Rails, there are plenty of examples of decently fast Ruby Rails Apps. Having 10 years saying they will improve performance and not getting any of it suggest problems with Gitlab itself more than RoR.