Hi, welcome to the forum.
I have a feeling some changes were made in respect of the Smart Preview generation, basically reducing the amount od CPU usage (in LR5 there were concerns that too much CPU utilisation was going into the Smart Previews build, which had a knock-on impact to other areas). But 5 times longer seems wrong, I'm certainly not seeing anything like that.
One thought: have you got the face recognition option enabled? Catalog Settings>Metadata Tab>Automatically detect faces in all photos. If you have, then all new imports are indexed in parallel with the Smart Previews creation, which will likely slow the SPs down somewhat. If you do have it enabled, you could either try an import with it disabled, or don't build the SPs until after the import has finished.
I've just built SPs for 100 raw files, took about 2 minutes on my old Win7 desktop.
BTW, there's a third option if you wanted to stay with LR5....use the latest free DNG converter to process your E-M5 II's files, then you can import the DNGs into LR5. But I'd try to fix the issue with LR6 first.