I believe github onprem gets around the speed by using jruby? I’m sure that could be done with GitLab as well.
As for the hosted solutions, IMO both GitLab.com and Github.com are pretty slow. I've just recently started using non-premise solutions of both and Github slowness is really taking it's toll on my patience.
For context: Ebay, Amazon and Google (Plus, Talk) are using Java for web. None of those are what I would consider slow.
If people were condemning Java for web as being slow, and Atlassian used Java and was slow, then you could reasonably assume that the reasons are correlated.
That is the case I'm making about Gitlab and RoR.
EDIT: you're mentioning Jira Cloud being slow, but we have the on-prem version and it's also very slow.