Bad FPS in Games

It could be a great many things causing it..
I know one commonly done mistake is adjusting graphic settings via nvidia control panel yet having graphics set to another setting in-game.. as this causes conflicts with priority.

Have you adjusted graphic settings via Nvidia control panel?

If so Id suggest reinstalling the Nvidia GPU driver and choose "custom installation" initially and choosing "perform clean install" and leave the settings at their default values.

Then use geforce experience to "optimize" (a feature of geforce experience) your games graphics to give the best settings for your hardware.

Also instead of removing geforce experience Ive heard that enabling experimental features then rebooting twice removes any negative effect it may have on gameplay.

@Lemurensohn

Since your laptop is pretty much just like mine, except that i have a 512 GB SSD, here are a few things I did to get optimal performance, in games and work (note that I am saying optimal, not max; for i do not believe in the term max).

Windows 10:
1. Disable updates; these kick in when you least expect them (there are multiple threads here on how to do that).
2. Disable windows search and superfetch services. For home use, you really do not need either of these.
3. Adjust power settings to your liking. Do NOT set to max performance unless you really feel the need for it. I am a power user, and I never use that setting. My power setting is fine tuned for putting CPUs to park mode whenever possible as soon as possible. This keeps my laptop cooler and increases longevity of the device too.
My power plan if you want to use it is attached, along with instructions on how to use it.

Software:
1. Use Throttle Stop (or something similar) to undervolt gently (mine is -125 for cpu and cpu cache) to reduce temperatures at high load.
2. Install a software like TuneUp Utilities to disable all optional software services (they get auto turned on when that app is opened, and disabled again when closed). This really helps reduce lots of background processing.

Gaming:
1. Set your FPS to 60 (you could turn on vsync too). Our screens are 60 Hz, so more than 60 FPS doesn't have any visual difference to us, except that it would be putting load on our processors.
2. Turn on certain settings in game that you feel not needed (like I turn off nvidia hair in witcher 3, turn off certain effects like God rays and such that I am not a fan of). Some visual settings do very little or negligible visual impact but put significant load on processor.
3. I removed GE Experience. I prefer tuning the games to my liking.

Hard Disk:
1. Since you mentioned your hard disk is 256 GB, partition it- Windows 10 should be good with about 100-120 GB, and the rest create a second partition for games. And use this only for heavy games. You can use the secondary slower hard disk for smaller or less intensive games.
2. Also, if you have a secondary disk, partition it, and put in the initial partition for games, next one for frequently used stuff... in that order. On normal spindle HDDs, the partitions inside get faster read write speeds (translates to the partition creation order, with the ones first created with faster speeds).
On mine:
SSD --> 1st partition Windows (118 GB), 2nd for games and virtual machines (remaining disk)
HDD --> 1st partition games, 2nd for movies n such, 3rd for Documents

Physical Unit:
1. Put in a couple of felt pads under the laptop feet to bump its clearance below to about an inch. This would allow for increased air flow below allowing the laptop stay cooler.

With these above settings, here is the user bench mark: http://www.userbenchmark.com/UserRun/9722798

And I get pretty much constant FPS in all the games I play with my temps ranging in 35-45 on idle to low loads, hitting max of about 75 during heavy gaming.

And on battery, with low to day to day tasks, I get about 3 hours.