Dear @Dan C,
I can confirm your observation that having an active selection (marching ants) is delaying the layer thumbnail update substatially. The thumbnails do update within a second after deselection.
My example settings are attached (I'm on Win11). However, the problem persists with all possible options of "Renderer" and "Hardware Acceleration".
Additional observation:
With Hardware Acceleration OFF, having an active selection (marching ants) is using significant amount of CPU power: 20-30% distributed evenly across the 32 logical cores of my CPU (each core is loaded to 20-30%). This is just to draw the marching ants, which again seems pretty excessive, and might indicate a problem.
With Hardware Acceleration ON, and marching ants the CPU is at 2% and GPU at 7%, but the thumbnails still don't update, until deselection (or after very long time).
So it's not the lack of available performance, but some other way the marching ants delays thumbnails update. Maybe the fact that on the CPU the marching ants utilize all available cores, without actually maxing out any them is an issue. If the thumbnail update runs with lower priority than the marching ants, then it's maybe blocked by it on every core?
