it "launches" .bashrc?

.bashrc is the session settings file for bash, that means its contents are loaded & executed everytime a bash shell is openend, so not only for login shells like .bash_profile.

"which will create the pid of the process inside /dev/cgroup/cpu/user"

You mean so that there is already a bash process running in the system that is used for all tasks according to cgroups from the moment of spawning via .profile, instead of ash? So basically all user tasks run under bash instead of ash? Sorry, i still don't get it. Maybe it's the language barrier too.

I do not think that bash is quicker, especially for login / system shells. That's for example why ubuntu (and god i hate that distro!) switched to dash (which is debian's version of ash, hence "Debian Almquist Shell) for the default shell, because it's lightweight in opposite to bash.

https://wiki.ubuntu.com/DashAsBinSh