Why does build lighting hang for 30 seconds before starting?

Swarm seems interesting, but relevant only after the 30-60 second UI hang ends. How do I open it?

The docs don’t seem relevant to the hang: Unreal Swarm | Unreal Engine Documentation

Attached is a screenshot showing Unreal’s not doing any CPU/Disk during the compile. I can’t “Stop Build”, move the dialogs, or the windows during this time.

I have shaders set to compile on GPU and CPU. I thought this might be the case, but why does the UI hang?

This scene is a demo scene from a Lowpoly woodland asset from the marketplace, on an i7-8700 w/ a 2060. The computer isn’t a monster, but it’s fresh.

> On 4.22 this has been happening a lot. It just kinda hangs.

A UI thread hang seems like a bug. Is there a log?