A little update on what has happened so far:
-Disabled every bit of fancy design/effect for Windows
-Disabled/Killed every service and process that he would not need...like ever.
-Basically stripped Windows 10 as bare as humanly possible (still can't get rid of Cortana though
-Tried the 8GB DDR3L module...doesn't work...going to trade that stick in and get him an additional 4GB module.
The change is actually apparent...even during typing on this site...before I did all that, the text would always lag behind by 500+ MS...this is no longer the case.
I can now open a youtube video AND have openOffice running at the same time without the system going bonkers (whoa! xD)
I don't think I can put Linux on this...he's the most basic user you can imagine...Linux would scare the hell out of him, I think. He's a grandma trapped in the body of a young guy.
I did tell him to get rid of the machine...but he doesn't want to. I told him I could get him something better...but I guess he's too scared ^^.
Also, turns out he paid $450 for a "set". The $5 keyboard+mouse and the VGA-only HD Monitor that he doesn't even use were part of the set.
Seriously though....do they even make VGA only monitors anymore? If naming and shaming is not allowed the mods can feel free to remove it...but the store is question is "otto.de"...some rather big German online shopping site...
Definitely not gonna buy anything from them XD.
Since the board also doesn't allow for any BCLK overclocking or anything like that...I think I have done everything I could. I thank everyone for their input!
P.S. Warframe now runs 1080P, all low, 30-60FPS, CS:GO and LoL are also playable. Since he doesn't care for graphic settings as long as he can play 1080P(big TV) he's happy.