Solved - it was the Firewall. I have Comodo Firewall on this computer and somehow Cura got marked as a "Contained Application". I don't know why I didn't encounter this when I upgraded to 4.6.0, but once I changed this, slicing was instantaneous again.
As an aside, Windows defender popped up a message asking to give Cura permission the 2nd time I rebooted the computer after this occurred. Again, I don't know why it didn't do that the first time, but that's what gave me the idea to look into Comodo firewall - along with my comment above about it being a security issue.