May 2021 updates slow Photoshop down, brushes now very laggy

I can no longer accurately use the brush tools when refining masks in Select and Mask. Since the May updates of updates of PS 2021 (22.3.1), then PS 2020 (21.2.7) there is 3-4 second delay between applying a brush and seeing the results. Switching off High Quality previews makes no difference.

I’m now running PS CC 2019 (20.0.10), which – thank god – hasn’t been updated, and which is much snappier.

I’m using Catalina 10.15.7 on a 5k iMac 3.5GHz quad-core Intel i5 with 24GB of RAM. A little long in the tooth, but it’s never had any problems coping with Photoshop until these May updates.

Anyone have any suggestions (other than wait for the M1 28-inch iMac release...)