Regular 504 Timeout errors on jenkins on some operations (e.g. node config updates)

Seen by myself and @steelhead31 and possibly others (@Haroon-Khel have you been having this regularly too - I know you mentioned timeouts a couple of weeks back)

I'm getting it 100% of the time at the moment when I try to save a node (machine) definition, although it seems to take effect immediately the page continues to spin until it times out. It does not block work from continuing in another browser tab.

Start by reproducing the 504 when saving a Jenkins node definition and compare the browser timeout with whether the change is persisted immediately. Done means the node save operation completes without the page timing out, including the reported configuration-update case.

Written by the indexing model from the issue text.