Operating system: Linux-5.6.0-1-amd64-x86_64-with-debian-bullseye-sid 64 Bits
Graphics card: Quadro RTX 6000/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 440.82
Broken: version: 2.90 (sub 2), branch: master (modified), commit date: 2020-05-18 09:05, hash: rBf3edff2d7d60
Worked: Never, not within the current subdivision surface modifier design.
Short description of error
In some models topology is compared incorrectly, leading to very poor performance by inability to re-use cached topology refiner.
The root source of this is coming from vertex sharpness comparison: comparison happens between "requested" sharpness by the user with "refined" by the OpenSubdiv. Thing here is: OpenSubdiv will assign veretx sharpness for loose, corner, and extra-ordinary vertices based on adjacent geometry, making comparison to happen between apples and oranges.
Exact steps for others to reproduce the error
Get file from T75579, open it, start playback. The performance will be very poor.
A bit hard to see whats' going on without debugger attached, but the openSubdiv_topologyRefinerCompareWithConverter() always returns false.