Meh, I run both on identical drives, and running desktop mode they are about the same speed. Games run the same fps. Benches score the same with admittedly a minor improvement on win10. But it's really minor.
My windows 7 was installed in may 2011, it's imaged and transferred to the present drive, while being in a dual boot with win8 at the time. Did the same with win10 which I first had on an old raid array and now on a bigger drive. Had to reinstall win10 due to a broken hard drive from seagate (10 days old) which had nothing to do with the transfer from raid.
Windows 7 boots faster in my setup, has all the bells and whisttles enabled, but that's all in desktop mode. My cpu stays at 1600Mhz when idle.
So there must be something wrong if your windows 7 is running wild.
As for games, sure, dx12 will force people in using windows 10. And it's not bad at all, minus the spying. Looks and works about the same as windows 7. SSD is also better on 10 and there are some other nice improvements.
Anyways, I keep windows 7 around since it's rock solid, and I got all my stuff on it like mail adresses etc etc.
Windows 10 still has the occasional quirk, especially after some major update like the anniversary upgrade.
And while it seems handy, it stores alot of your adresses and settings, it also feels like giving your home key to the neighbours.