Constant error messages "AT-SPI: Could not obtain desktop path or name" cause poor performance

I just noticed today with mutter master the gnome-shell login animation stutters badly, as does dragging windows.

Bisected (I don't understand why, but this is indeed the problem):

bd624e4dfb4de728b5cd8ed9e5fd2d6d6104b84f is the first bad commit
commit bd624e4dfb4de728b5cd8ed9e5fd2d6d6104b84f
Author: Jonas Ådahl <jadahl@gmail.com>
Date:   Mon Nov 12 11:29:44 2018 +0100

    tests: Override X11 display number
    
    As with the Wayland display name, to avoid clashes with already an
    running Xwayland or Xorg instance, override the X11 display name to
    something less likely to cause a clash.
    
    https://gitlab.gnome.org/GNOME/mutter/issues/193

:040000 040000 7fcbd06cf416269645cc96a98036bf2d6aff17f3 1eecb43a92f7ee0bed5d35067008eb6a61dc5f35 M	src

Performance results on an i7-7700:

• bd624e4d stutters, and 113% CPU to drag a window.
• fe1d9fb4 smooth, and 38% CPU to drag a window.