The subject line says it all! Since the upgrade, the machine is practically unresponsive (KDE login takes 10+ minutes, no exaggeration, to get from the login screen to the desktop) for at least an hour after every bootup. Remote login confirms "md126_raid5" and "md126_resync" processes using a large chunk of CPU. (/dev/md126 is the hardware RAID.)
The first time it did this, I figured there must have been a file system update between 42.1 and 42.2, and that this would be a one-time update. I could tolerate that. But every single boot?!
Any suggestions (short of reinstalling from scratch) are welcome.