You must be using the New Assembly Constraints, because if Mating Conditions was your default, there is no 'Move' Component option (it's titled 'Reposition').
That said, is your assembly large? Now I just tried this using NX 5.0.4.1 on an assembly with over 11,000 components and while it did take perhaps a second or two longer for the dialog to come up than when I tested this on an assembly with only a dozen of so components, it was still fast enough that I wouldn't have thought twice about it. Now there is one issue, if it is a large assembly with a really deep tree (many sub-assemblies inside sub-assemblies inside sub-assemblies, etc,) when you move a selected component that is part of a sub-assembly, the system has to first set that sub-assembly to be the Work Part, which can take a little extra time, but again, in my 11,000 component assembly, even if I pick as deep as I can (at least 4 levels down), it still only adds a second or two at most to the dialog coming up.
One more thing that it might be. If you have lots of inter-part relationships and you're using Partial Loading and you're NOT loading inter-part date, if you are selecting components several levels deep in the assembly structure, it is possible that when they set the sub-assembly to be the Work Part they extra time will be taken to fully load and perhaps even update inter-part links that will also get loaded at that instant under those conditions.
Anyway, unless it's some sort of inter-part updating thing, I can't think of anything that would case a 'Move Component' command to be anywhere near as slow in coming up as you seem to be telling us that it is.
John R. Baker, P.E.
Product 'Evangelist'
NX Design
Siemens PLM Software Inc.
Cypress, CA