A DNS or firewall problem that causes Gitea to intermittently spend 10+ seconds responding to requests, when running in a cluster alongside other containers that have no such problems? Can you describe a plausible mechanism for that?

We did some profiling and spent quite a bit of time looking at Gitea's source code, it was pretty clear in the end that it's just very very inefficient for large setups. It does an excessive amount of I/O on the Git repositories every time you load a page; there is some caching but not enough / not of the right things. We were really open to implementing fixes and submitting PRs but the community was so hostile that we just abandoned it.

It was overall an enormous waste of time and I can't recommend Gitea to anyone with a setup larger than a handful of small repositories.