Did you stop the Windows Defender too? Or any other anti-virus? Or configure them to ignore the files in your PlatformIO installation – not just the oroject locations.
To really get a grip on what Windows processes are doing, you might find Process Explorer handy. It’s at Process Explorer - Sysinternals | Microsoft Learn there are other handy utilities by Mark Russovich, namely File Explorer and Registry Explorer, where you can set a filter on a program you are interested in, and have the utility show you everything related to it.
One or more of those might give you a clue. I’m a Linux user, but a while back I installed PlatformIO on a Windows 10 laptop and didn’t see any sliw downs. The laptop being about 16 years old, dual core with 8 G of RAM, so nowhere near today’s top models.
I used to work in support roles, and those Mark Russovich utilities ere the bees knees in helping diagnose problems. Microsoft were also impressed and bought them, and gave Mark a job to boot!
HTH
Cheers,
Norm.