How long did you wait before giving up and killing SketchUp? There is a follow-up phase during which SketchUp converts the imported geometry into its own internal representation and does cross-checks to deal with things such as duplicated vertices, coplanar faces, etc. That stage can take a long time - hours even for a complex dwg. I can’t explain why SU 2017 would take longer than previous versions, but you should at least give it chance.
The STEP importer is not a Trimble SketchUp product; simlab is not owned by Trimble or part of SketchUp. Unless the import is causing SketchUp to crash, there really isn’t much Trimble can do to help you. That’s not to say that no aspect of SketchUp 2017 could possibly be to blame, rather that it is up to simlab to diagnose why their plugin is misbehaving.