If you’re on NixOS unstable, then GNOME Files (Nautilus) will become very slow, as well as a RAM hog, once you upgrade to a version newer than about 2025-11-03. The system journal (journalctl) will show logspam along the lines of
Nov 18 01:23:45 localhost org.gnome.Nautilus[22570]: While adding page: duplicate child name in GtkStack: Documents
This is a known issue, Nautilus Very Slow When Setting Templates Folder to "$HOME/" (#4014) · Issues · GNOME / Files · GitLab, and should go away once Glib 2.86.2, containing Backport !4886 “gutils: Strip all trailing slashes” to glib-2-86 (!4890) · Merge requests · GNOME / GLib · GitLab, is released and @jtojnar pulls it in.
In the meantime, the workaround is to open ~/.config/user-dirs.dirs and replace
XDG_TEMPLATES_DIR="$HOME/"
by
XDG_TEMPLATES_DIR="$HOME"
that is to remove the trailing slash from that variable. You may also have to manually kill all extant instances of .nautilus-wrapped afterwards.
(If you’re reading this in the future when Glib 2.86.2 has already been released and are still observing a similar-seeming issue, then it must be something else.)