Its default is set to "-1" which sets Core to use all minus one of your CPU Cores.
I don't have direct experience with this issue with Bitcoin Core, but:
* In my experience with various other CPU intensive software I use (more than average) max - 1 usually leaves Windows quite responsive.
* Also keep an eye on the disk usage, since in my experience, if CPU is already at max - 1 then the throttling happens because of the disk (PS. is the blockchain on SSD or HDD?)