Microsoft says it's fixed an issue in Windows 10 Game Mode that 'results in lower frame rates'

If you are still experiencing gameplay issues in Windows 10—and specifically stuttering and lower than expected frame rates—hang tight, a cure for what ails your PC is coming. Hopefully, anyway. It’s worth remembering that it was a previous Windows update that caused these performance woes on some PCs in the first place.

Unfortunately for some unlucky users, the cumulative Patch Tuesday update Microsoft rolled out in April had a penchant for trashing frame rates and causing games to stutter, without any obvious rhyme or reason.

source: gamezpot.com