As per title. A post on NixOS users’ Discourse implicated XDG_TEMPLATES_DIR, although that one was about a bug in Glib newer than what’s in Ubuntu 24.04, involving trailing slashes. But I had mine pointed to $HOME/tmp, and apparently Nautilus tries to read all the garbage files I have therein, almost choking to death while doing so.
I made a separate, new (empty) directory, pointed XDG_TEMPLATES_DIR to it, killed all existing Nautilus processes and hallelujah! It’s no longer slow.