You can apply Ground relationships to parts to keep them from moving in an assembly.
Why are commands extremely slow? If I remember from another post, you are using a very expensive and powerful Boxx workstation. There is likely something about your design process causing you headaches that you should fix..
Well, fully modeled threads will certainly hurt your performance if you have a lot of them in an assembly, but it appears as if you are having serious graphics display issues outside of this.
While in a part or assembly, open up the file menu, select Solid Edge Options on the bottom left of the dropdown, and go to the View tab. On the right side, what does the Application Display dropdown box have in it? (Graphics Card Driven, Backing Store, Software Driven, etc..)