I searched the topic and didn't find anyone who was having the same problem.
I just imported a massive STEP file. The .stp itself was 250MB and it took about half an hour to load...
But that's not my problem. Once all the parts assemblies had been created, I got a dialog box that told me it was suppressing all of the assemblies. There were probably a hundred of them and I had to go through the design tree and unsuppress all of them.
I checked the import settings and there doesn't appear to be an option related to this.
Is this something I can prevent?