Spacemacs was completely unresponsive at startup. Turns out, for whatever reason, it was after I was using my laptop in a new place, and based on this FAQ obviously the ISP there was doing something funky: https://github.com/syl20bnr/spacemacs/blob/develop/doc/FAQ.org#why-is-spacemacs-hanging-on-startup.
Adding the settings there to the user-init function did the trick:
(setq tramp-ssh-controlmaster-options "-o ControlMaster=auto -o ControlPath='tramp.%%C' -o ControlPersist=no")
