WOW, yeah thats not right, i have lower specs than you, (i5 3570k, 16GB DD3, GTX 1060 6GB) and i have never seen Bigbox anywhere near that bad. I would have to agree with what other people have said in here that there i s program/AV on your system interfering there some how. Have you tried disabling AV software while running Bigbox? does it behave the same if you do? If its still like that with AV off it could be another program interfering but narrowing that down could prove difficult. Your specs are very very good, so it has to be something specific to your system.
EDIT:
Also i have around 15,000 games in my setup, with perfect performance, so the fact that you get this performance with only 40 games is very strange.