System Information
Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: GeForce GTX 1060 6GB/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 442.19

Blender Version
Broken: version: 2.83 (sub 2), branch: master, commit date: 2020-02-01 16:56, hash: rBf051d47cdbce
Worked: it never worked good!

Short description of error
sculpting with a shape keys on is laggy.

Exact steps for others to reproduce the error
1- make an Ico sphere with 8 levels of subdiv (just make a dense mesh)
2- add a shape key (1, 2, does not matter just adding a Basis is enough)
3- try to sculpt with shape key being on or off (0 or 1 or even on Basis)