Those are relatively large model files that could potentially contain millions of edges and faces. How well structured are they, that is, are all their edges and faces captured inside groups or components, or are many of them “loose” in the model? If there are a lot of loose edges and faces, SketchUp has to merge all of them during the import or copy and that could take a long time. On the other hand, if there are a lot of components it can take a long time to create the thumbnail images for the component browser.
Either way, given your pretty massive computer an hour seems like a long time.
Boy! With 128GB of RAM I’d really hope it isn’t paging or swapping due to SketchUp! But that raises the question: what else is running at the same time? If you have some other memory hog also live, it could be that even that vast memory is being overbooked. Have a look at the task manager while SketchUp is hung to check on this possibility.