With Windows 10's launch, all three major PC graphics companies (that's Intel, Nvidia, and AMD) released updated drivers for the new operating system. Both Ars Technica and Maximum PC have taken the new drivers for a spin and have reached the same conclusion: gaming performance in Win10 is largely the same as its predecessors.
Ars Technica found that video cards from both AMD and Nvidia performed similarly to how they performed in Windows 8.1. Their tests used a Core i7-5930K with a Radeon R9 290X, R9 295X, and GeForce 980Ti. They tested with Windows 10's Game DVR feature enabled and disabled, too, and found that Game DVR does excise a small but consistent performance tax. In the end, Ars found that it is safe to upgrade to Windows 10.
Maximum PC's tests used a single configuration with the same Core i7-5930K model and a GeForce 980Ti, but the upgrade process wasn't smooth—you could even say Something Happened. They tested a different set of games across Windows 7, Windows 8.1, and Windows 10 and the results were similar. In every test, Windows 10 performed within a couple of percentage points of its ancestors. Maximum PC also concluded that it is safe—from a performance standpoint, anyway—to upgrade to Windows 10.