Performance issues on isoworker*.dragon results in automated test suite failures

So tails#21032 is still not fixed. That ticket is very focused on failures to restore VM snapshots in a timely manner (within 30 seconds) but there are other failures too, so it seems dragon has general performance issues that affect a lot of stuff, triggering timeouts.

In particular it seems that the recent disk failure (#18215 (closed)) is unrelated to the performance issues.