There really is no need to run CleanUp while shutting down and while booting.
One of them is enough.
You can have avast load before there is a internet connection, but that would mean avast will not get the latest detection definitions when you boot the system.
I strongly advise to leave it as it is.
For the slowness, first thing to do is checking what is loading at boot time and running (could be things in the background)
Disable everything that is not really needed for the system to work.