Halting, choppy play in Arreat GRs

As the title says, I experience very choppy play in the GRs that use Arreat Crater as maps. All other GRs are smooth and chop-free. Only the Arreat ones stutter…and the stutter becomes worse, the further into the map I go. Once I exit to a new floor, all stutter is gone. Why only this map?

Normally I might have expected it to be on the hardware end of things but my latest PC is a brand-spanking new powerhouse running an NVidia RTX 3090 card, 64 Gigs ram, SSD, etc. Only Diablo is installed, nothing else.

Is anyone else experiencing this stuttering? Better yet, does anyone know how to fix this?