Okay, I figured it out, and it's partially a hardware problem. It turns out that LR is scanning my USB hub (which has an empty card reader in it), and for some reason that takes LR about 8 minutes to execute. If I disconnect the hub, the LR problem goes away. Note: there doesn't appear to be a problem with the hardware itself, just in the way LR interacts with it.
It looks like I'll be adding myself to the list of people who wish there was a way to exclude file locations in LR. There doesn't seem to be any way to address the problem (although I would really like to know why it only started when I switched from LR4 to LR5). :-(