Harmonize layer mask is incredibly slow

I have noticed something similar with AI-generated masks in Photoshop. The generation itself can be reasonably fast, but editing the resulting mask with the Brush tool can become very laggy, especially when trying to make small precise adjustments.

It might be worth checking whether the issue is related specifically to GPU processing or the complexity of the generated mask. If possible, testing the same mask after rasterizing or simplifying it could help determine whether the mask itself is causing the input lag.

Since you're on an M1 Ultra and the rest of Photoshop seems to respond normally, it does sound like something specific to how the Harmonize mask is being handled rather than a general performance problem.