ksmserver autostart had a system where processes, such as plasma, could
block the startup phase from moving on.
It is currently entirely unused.
It's very difficult to use properly, plasmashell has to block startup
before ksmserver moves on to phase 1. Racey at best.
I did hack it to work, and it does make a surprising positive
improvement to bootup. Worth exploring the idea properly, but implemented
better.
We should be able to get the same results by
blocking and waiting for the DBus name in the autostart desktop file to
become registered rather than having a ksmserver-specific solution.