Frame stutter/Low FPS in windowed fullscreen,no problems in exclusive fullscreen

[Thunderysteak]

I'm trying to figure out a really weird issue for my super tech illiterate friend and their laptop. The game they mostly play (WoW) has removed exclusive fullscreen and all of the games that do not have exclusive fullscreen have crazy frame stutter or generally awful performance, but not in exclusive fullscreen. I've been trying to fix it through teamviewer so I can't really tell what kind of laptop is it.

Specs:
Laptop: OEM Design based of either CLEVO P670RG or SAGER NP8678 (mobo is CLEVO P65_P67RGRERA)
CPU: Intel Core i7 6700HQ @ 2.60GHz
GPU: NVIDIA GeForce GTX 970M with 2GB VRAM
OS: Windows 8.1

From everything I've searched, everything says it's a G-Sync issue, but the laptop doesn't come equipped with G-Sync display and G-Sync is not displayed in the nvidia control panel. I fully updated and patched the laptop, reinstalled the Intel igpu and nvidia drivers, disabled dpi scaling for those games, made sure everything else is turned off when games are running, even disabled superfetch, made sure it's not the HDD that's causing the lags, disabled fast boot, made sure it's not overheating (and is actually running in turboboost) and ran virus scan.

I can't really figure out what the absolute fuck is wrong with that laptop and I've been ripping my hair out for days trying to figure out the root of the problem. Maybe it's actually a hardware issue? No they don't want to upgrade to Windows 10.

[Benzine]

Does he have Nvidia Optimus on it? Could be switching to some desktop low power igpu mode. Can try forcing the main gpu via this method.

[Thunderysteak]

Yeah that was already done
I even checked with GPU-Z that the main GPU is being used for games by looking at the performance usage
I might try to pressure them into upgrading them to Windows 10 by buying them an OEM key if I can't figure out any other alternative

[Thunderysteak]

Okay holy shit I figured it out
It's been the CPU being dumb with turboboosting
I basically set in the power options the CPU to utilize only 99% of it to force the CPU to not turboboost and that fixed it