Prior to making those changes, did you look at loading your assembly lightweight or simplified? It sounds like you may have created work needlessly.
How large is the assembly? Are you opening across a network? While it's not as much of a problem as it used to be, it may slow things down.
The assembly is about 750Mb, we work out of a working folder on our local drive. The bulk of the lag time is simply due to processing all of the files. And rebuilding everything. My computer has a SSD, and I have even tried loading the files into a ram drive, but that only gained several seconds of load time.
And yes, I have tried lightweight components, but the effect is negligible, especially compared to the part file assemblies.