So simply disabling the Corel Update Helper wasn’t enough to get my machine back in reasonably working order; I couldn’t figure out what was causing CUH to relaunch periodically and simply disabling the LaunchAgent didn’t do that. So, I ended up deleting CUH entirely:
sudo rm -rf /Library/Preferences/com.corel.* /Library/LaunchAgents/com.corel*
and while I was at it, I uninstalled Corel Painter.