Depends upon game to game. If you are talking about Assassins Creed Unity, yes it is terribly optimized. Your PC specs seem quite good. If you are facing these issues with all the games new and old, issue might be with CPU, GPU or windows.
Try monitoring temperatures uisng HWmonitor or some other utility, CPU should be below 70 and GPU should be below 85-90 degrees. You may also run stress tests using OCCT on GPU and using Hot CPU tester on CPU to identify if any of these is malfunctioning.
If everything is working fine, the issue might be with OS, run msconfig and disable all startup items and services after hiding microsoft services and restart computer. then try running games and see if it helps. If games work fine, there might be some application that might be interfering, enable one item at a time in msconfig to identify the culprit.