System Information
Operating system: Windows 10
Graphics card: 900 Series

Blender Version 2.8 (96f762500ef9)
Broken:
(example: 2.79b release)
(example: 2.80, edbf15d3c044, blender2.8, 2018-11-28, as found on the splash screen)
Worked: (optional)

Short description of error

Exact steps for others to reproduce the error LAG when shifting dyntopo on and off with subd surfaces loaded.

1. Make icosphere
2. Add SubD modifier. Leave default. Don't increase. Go Sculpt Mode
3. Try the default draw brush on icosphere. Should work smoothly.
4. Activate Dyntopo. Use brush on icosphere again. Should work smoothly.
5. De-activate Dyntopo. Use brush on icosphere again. Noticeable lag when using brush again.

Verts/Faces/Tris < 4K
Mem:23.8MB
v2.80.40

Thanks.