Group::ChildrenController#index.json / loading a group page is slow
### Summary
Even if the group page has been visited before and not changed meanwhile, it loads very slow (6–10 seconds).
### Steps to reproduce
- open group page
### Example Group
https://gitlab.com/gitlab-org
### What is the current *bug* behavior?
Group page loads slow despite the Ajax call returning `304 Not Modified`:

### What is the expected *correct* behavior?
In my opinion a page that has not modified should load in less than a second.
issue