tf2onnx is super slow using Python 3.8 and 3.9 on Windows

[TomWildenhain-Microsoft]

Use python3.7 for a more enjoyable experience on Windows. We are still trying to identify the cause.

[TomWildenhain-Microsoft]

Extreme slowness is caused by lack of cpp accelerated protobuf on Windows for certain python versions.
protocolbuffers/protobuf#7967

[TomWildenhain-Microsoft]

Protobuf has solved this with a cpp release for windows. Solve with:
pip uninstall protobuf
pip install protobuf