In Unity 2021, I found a major stuttering problem, to the point of unusability, when painting terrain. Specifically, as the brush moved, the console filled up with “Allocating 56 bytes” messages, and quickly had 999+ messages (that being as high as the counter went) and every operation, terrain related or not, was very slow.
After some googling, my solution was in the Terrain settings, lower the Detail Resolution. IIRC it was at 1024 but I changed it to 256. That still wasn’t enough: the damage was done. I had to quit Unity and restart it. THEN it speeded up.
If you really want a high-res terrain, probably should make it in some 3rd party tool (l3dt, worldmachine, terragen, terraforge3d, or even Blender!) though getting the result into Unity might require some research.