On the plane to Edinburgh I got to the bottom of a persistent "launchd is eating 90% CPU and IDEK why" problem, which is probably a first in recorded human history and which I feel pretty great about.
The deal was: Launchd freaked because I copied a user account's files from another computer, where that user had a different name. There were some service plists in
~worknick/Library/LaunchAgents (Spotify web helper, and something involving Steam) that were trying to run helper apps stored in
~nick/Library/Application Support, because "nick" was worknick's old name, but he already existed (and was a different person) on this computer, and worknick couldn't access those files!
For some dumb reason, launchd didn't have a cooldown between tries or anything, and was trying to restart these un-startable jobs many times a second. I edited the Spotify plist, killed the Steam one, took out a few old ones that weren't relevant anymore, then rebooted. SOLVED. Amaze. Now I've got something like quintuple the amount of battery life when the worknick user is logged in.