I must say I'm a bit reluctant to downgrade to Windows 8 because I fear I might miss crucial Microsoft updates, which may become a security problem. But maybe it's a good idea, because I haven't yet discovered the benefits of Windows 10.
By the way, setting the CPU parameter to 1 instead of 0 in GPxPatch seems to help. The game is much faster overall, but still it gets slow from time to time. I guess it might be a processor problem. I believe in the new GPxPatch the default CPU usage is 0, because selecting "all" may cause the game to crash, so I hope 1 is still safe. So, all in all, there is some good news and there is definitely hope for the future, but if someone is able to diagnose the problem, I would love to hear it!