The "TCP/IP NetBIOS Helper" service is definitely causing this problem. Disabling it allows me to boot very quickly. Setting it "Automatic" or even "Manual" results in a black screen with cursor, lasting approximately 2 minutes.
Apparently "Automatic - Delayed" is not an option with this service, as it complains about the flag option. If I disable it, I can then start it from services.msc, with no delay whatsoever.
Nothing really special about my network. Just another homegroup laptop, a wireless printer (with an SMB share), and some UPnP devices. Interestingly, fileshares are NOT visible unless "NetBIOS helper" is running... but I'm not going to wait this long every time I boot up.