Thanks for your replies:

I haven't seen anything suspicous in the task manager & resource monitor. Its mostly Firefox and Gs3d or Unity. I still have lots of free ram too. Also CCleaner didn't found much, just some temporary files etc.

Its also doesn't seem spyware related, since it happens quite predictable after sometime using programs like GD3d or Unity (never e.g. on startup / restart or the first few times using these programs). I have found topics though of people having problem with page files on Windows 10, e.g. like this: http://www.ibtimes.co.uk/windows-10-how-fix-sluggish-performance-boost-overall-system-speed-1516413 . So far the topic's solution works (although it is a bit soon the tell), hopefully it will stay that way.