I purchased Lenovo Legion Y720 a week ago. For 2-3 days all games worked pretty good i used to get 60 fps in High settings in games like GTA 5, Mafia 3 and warframe. But now i'm getting like 15-20 fps. Things i treid:
Updated my drivers, Reinstalled Geforce experience nothing worked. I found that dxdiag command could not identify my total size of GPU. Check the screenshot maybe you will understand. 
FPS drops in new laptop?
Gahaninadh
Open Task Manager and see what else is running…
one thing I've observed is that the antivirus/antimalware system runs in background for like five minutes after boot up in order to check every file, etc. As you add more games, the total number of files to be checked goes up, up, and up!
Spock (rhp
That's the thing about some of the GPUs is that they will run really well until there's something that they don't like processing. Things like explosions, which produce a lot of particles to keep track of and draw. Cleaning out your harddrive isn't a bad idea either, do a little defragging and things like that to make sure when your game loads new assets it doesn't die trying to find it.
There's a ton of optimization you can find on /r/pcmasterrace, places like that.
joseph