If you are a customer you can file a support ticket and our Support Engineers can help troubleshoot. One of our mandates is "performance is a feature" and we want to better understand what's happening in the wild.
I'm a community user, and the sluggishness seems to be a common sentiment so I'm not sure profiling my instance will add anything valuable that's not already known.
If not, and you are interested, we built a tool we use called "fast-stats": https://gitlab.com/gitlab-com/support/toolbox/fast-stats
it might be able to pinpoint what controllers are slow for your own curiosity.