I just got a new disk for my comuter, and installed windows 10 from scratch on it. in the first weeks everything was fine, and I was able to play any game with ultra settings. But after a few weeks everything which needed good graphics just got slower. Now my computer even struggles to play games like fallout 4 on the lowest quality possible.
I have already updated my drivers and it doesnt fix my issue.
Open task manager with CTRL+SHIFT+ESC and sort by CPU usage.
Maybe you installed sth. that is running in background and silently hogs resources.
Also Windows dislikes if you fill your hard disk more than 85%