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After months of reports from users, Microsoft seems to have finally acknowledged the issue. As it turns out, there’s some sort of bug in the Windows 10 Creators Update that’s causing the FPS drops.
Disabling the Xbox Game DVR feature has proved to be a solution for some users and Microsoft have said that a fix is due for release probably with the Fall Creators Update.
Microsoft finally gets around to fixing Windows 10 Creators Update gaming performance issues - MSPoweruser