GitLab employee here. As much as it pains me, I have to agree with you for the most part. Performance is one of the biggest pain points for our frontend team, but is something we're actively working on.

Here's one of our epics to show our progress: https://gitlab.com/groups/gitlab-org/-/epics/716

We're in the process of migrating parts of our codebase from haml templates to Vue apps, which does have the unfortunate downside of requiring javascript to run. We're looking into solving this with server side rendering, but it's early days right now.

Here's the issue for that: https://gitlab.com/gitlab-org/gitlab/-/issues/215365

If either of these things are things you could help with, we encourage you to contribute and help us make GitLab a better place.