From: https://www.freedesktop.org/software/systemd/man/systemd.special.html#session.slice

All essential services and applications required for the session
should use this slice. These are services that either cannot be
restarted easily or where latency issues may affect the
interactivity of the system and applications. This includes the
display server, screen readers and other services such as DBus or
XDG portals. Such services should be configured to be part of this
slice by adding Slice=session.slice to their unit files.

Slice=session.slice should be used{F2793696} in the relevant units under doc/examples/systemd-user.