Then it’s probably the environment setting. The noisy one uses v6 default and the clean one uses a v5 style.
But 80 Mb was big though… type purge and hit enter. That will clear out any block definitions too.
I presume it is the code for opening IGES vs STEP that causes this. DWG defaults to V5 style too. @pascal can you verify that? And it would be handy to have a “set V5 style environment” in V6 since often this yields a higher fps on complex files.
It is one of the things I have wondered about for a long time, but never gotten around to address. And Pascal do you know why is one shading like V5 when both seems to have the same environment?
Well… AFAIK there is no “make a v5 style environment option” so then the solution would be to open the “clean.3dm” file, delete the sphere and then IMPORT the STEP file into that one. You might want to double check the file tolerance and unit size first though.
Or copy-paste the content from one file to another. You can have two instances of Rhino open so it is easy to copy-paste from one to another.