That is a lot of parts, 4k parts and if you are then trying to show 3 versions of that to total 12k parts i can see that being slow.
It sounds like you are starting in Creo. I would try creating a simplified rep and reducing your sub-assemblies to "Envelope Reps". Basically if you don't need to deal with the individual parts of these sub-assemblies you can reduce them to one part. To do this create a new simplified rep view and then switch the sub-assemblies to "Default Envelope Rep", this will step you thru creating a new part to house the envelope, in my test case you can just click thru OK on all of these. Once you have done this for all the sub-assembplied you can "Erase Not Displayed" (not 100% sure this step is necessary) and then you can export a .pvz.
If you have changes to your assembly these envelope reps should update automatically and you can re-export.