there is a difference between scaling a rails application when you are running a cloud business and running a packaged application on-site. I'm pretty sure nobody is running gitlab backed by databases like AWS RDS or something.
I have shipped large rails apps as packaged software for on-site install (using jruby). It's not easy.
I think the point is that Gitlab faces unique challenges because anyone can host Gitlab on their own hardware, so they're always trying to solve performance problems in a way that any typical user could also solve it.
Github on the other hand can use strategies solely targeted at their SaaS performance. It seems that Github also maintains more control over their enterprise version than Gitlab but I'm not 100% sure there.
https://github.com/github takes ~2.2 seconds, but by about 1 second has most of the information on screen and is simply waiting for the activity graphs.
I have shipped large rails apps as packaged software for on-site install (using jruby). It's not easy.