Welcome to the forum 
The plymouth-quit-wait.service is waiting for something, it is not preventing anything iirc. Flushing your logs seems like a sane thing to do, if the log gets to large limit it to something you deem sensible?
If you could add info about the hardware you have other users might have an idea where to start looking to improve your boot times. Please provide the output of:
inxi --admin --verbosity=7 --filter --no-host --width
This thread lists some options to boot faster in gnome and seems relevant to your situation