This is not an intermittent or “only on startup” issue. We have multiple workflows and this one in particular always takes a significant time to load
Hmm, this sounds like a different issue to what we thought the OP was about. Do you see the red “you are offline” banner while it is loading? In fact what you are describing sounds more like this, assuming you do see the red banner appear: Disconnects in Cylc UI logs
if we try selecting a different workflow while this workflow is loading, it breaks the GUI for ALL workflows for that user until that initial request finishes
What do you mean by “breaks the GUI”?
It would be helpful to provide some screen capture GIFs of what is happening, if you are able (or just screenshots).