Downloading took about 20 seconds.
Update also got stuck at 10% for about 3 minutes (TOP process was "xz," so it was decompressing the downloaded file at that point.)
Upgrade step took a total of six minutes to apply.
Took about 90 seconds to reboot, and about another 2 minutes til the "Alert" message about the reboot came to e-mail.
About 15 seconds after that, the UI re-appeared...
Twonky was immediately busy re-indexing my MP3 and video collection... That took about 6 minutes.