In any case, this is a great topic and great test!
Indeed in most cases what is most directly important when opening an IFC file is the geometry. All the rest (attributes, etc) could come basically "on-demand".
That's basically the path I'm following with the multicore IFC importer. The speed gain over the normal IFC importer is already huge. If you run it without the GUI, it's even faster. My current preferred workflow is: 1) import an IFC file in no-GUI mode, 2) in a clean file, create Arch References to the main container/building of the file created in 1).
The objects created that way are non-parametric Arch components (although with all their IFC properties and materials). It's quite fast. I would like to go further that way, maybe somehow tie each object to its original IFC file and allow to "decompose" or make them editable (recreate the extrusion, etc..) later on? Still a lot to test/try there.
The 1) + 2) procedure above could also be condensed into some automatic tool...
The IFC explorer tool in the BIM workbench follows the mesh idea: It creates a mesh version of the model, which is quite fast, then you can choose which pieces to import individually.
In any case, those are many ways to reduce import time in a way or another.. Certainly we can do much better than revit/archicad!