My guess that it has something to do with Windows updates (maybe security related like patches for Meltdown and Spectre vulnerabilities that I've disabled) and recent (last few years models) CPU. I've made
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where in cwebp (currently for main RenPy [not bulk] compressor only) "-mt" flag was removed (compression speed for me without it the same but ur PC should become less laggy, at least I hope so) so can u try with it (use it only for tests) and revert back
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too.