Not saying I dislike gitlab, I actually really like it and we use the on-prem gold edition licensed to 6k seats. (as in, I put my money where my mouth is when I say I like it)
But of the things people complain about regarding rails that's that it's: large, heavy, slow et al.
All of those points carry directly over to gitlab and are the biggest argument against using the product.
I say that having seen a ton of PHP code base that is not stellar in itself, but has the more critical parts well optimized, going through external C libraries or DB hooks on parts that really warrant it.
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.
When our Co-founder and Engineering Fellow Dmitriy Zaporozhets decided to build GitLab, he chose to do it with Ruby on Rails, despite working primarily in PHP at the time
GitLab CEO Sid Sijbrandij thinks his co-founder made a good choice:
"It's worked out really well because the Ruby on Rails ecosystem allows you to shape a lot of functionality at a high quality," he explained. "If you look at GitLab, it has an enormous amount of functionality. Software development is very complex and to help with that, we need a lot of functionality and Ruby on Rails is a way to do it. Because there's all these best practices that are on your happy path, it’s also a way to keep the code consistent when you ship something like GitLab. You're kind of guided into doing the right thing."