10 is slower than 8.1, but faster than 7. The speed improvements in 10 were made in 8, and 10 is slower because it carries more baggage, as Microsoft knew they had the extra speed they could spend in the back end with things like telemetry and other processing that really isn't needed to run a computer, and users upgrading from 7 would still see a modest speed improvement, not realizing 10 was slower than the immediate predecessor, which is 8.1.
As far as gaming is concerned, Windows 7, 8.1, & 10 are going to run your games at the same speed, with the same hardware, with the difference being only the margin of error in measurement. Games utilize the DirectX API, and the most popular versions of DX are the same between these versions of Windows, so performance remains the same.
The only advantage Windows 10 offers gamers is the newer DirectX 12 API. Newer games can utilize this to their advantage, or target this API exclusively. When this happens, for best results, or running exclusive titles, picking Windows 10 will be a non-choice.