There is no valid reason either from the machine point of view or even the number of parts. I commonly have assemblies with 5000 pieces or more and I don't have any particular problems.
Check this out and let us know if you have these same settings in Tools==>options==>Assemblies==> Opening complex assemblies.
Also tell us if you have moving parts (flexible subassemblies) or if your ASM is completely static.
Creating speedpack configurations in the subassemblies and using them in the head assembly seems to me to be the most effective for a performance gain.
The screws are also very heavy depending on how they were created. The best thing to do is to create configurations without screws in the sub-assemblies to be able to work more easily in the head assembly.
You can also create lighter configurations, by removing the subsets that you don't need to work. While keeping the complete configuration...