Windows Defender - Very slow scans

Upgraded a laptop to windows 10 Home x64 for a family member. It's an older Acer Aspire laptop. We attempted to do a Windows Defender quick-scan to try it out. However, three hours later, it's only ~50% complete. I've never had a fresh install take this long for an AV quick scan.

My initial thoughts were..
1) Is the CPU being bogged down? no, only ~80% max during the scan.
2) Running out of available RAM / swap space? no, only about 60% RAM was being used during the scan

It's a fairly fresh install (couple weeks old) so I don't think it could be drive fragmentation. However, the computer is kinda old so could it be a failing drive? Am I missing some setting?

That's pretty strange. We all know laptop harddisks are usually slower (5400rpms) but 3 hours is an insane time for a quick scan. Can you check process manager to see if there is anything else running that is slowing things down?
On my desktop i7 (SATA II hd), Defender takes about 5 minutes for a quick scan.

Are you leaving Defender open when scanning? I found (a glitch maybe) that leaving it open gives a false timeline. Mine was showing around 50% complete after about 15 minutes, but I closed it, and reopened it, and it was done. I did it again to see, and it seems the display "sticks". The second time I just opened an explorer window, and then click back to Defender, and it was around 90% done in that split second. They were both on quick scans.