Hello!
0.5s freeze is not a micro-stuttering but rather a serious freeze. Of course, if there was an actual issue with the game we would have noticed it long ago. I don't recall any reports of this issue since the game was released in 2015. We've played the game ourselves and never encountered it too (I've just run it and tried pressing Shift while flying as well as in the inventory menu, no stuttering whatsoever).
It's reasonable to assume there is something else causing the issue in your case. Unfortunately, I don't have any suggestions except the usual steps (update the drivers, check third-party software that might interfere with the same).
Regards!