if it was a 95/5% split, that might be a fair (and terrifying) thing to say.

Mine is running under 100% control of grub

grub doesn't control anything, it transfers control to the OS and is never contacted or heard from again. More concretely, in the case of Linux, the init task (first task and parent of every other task) function has a "tail" call to cpu_idle which never turns.

http://lxr.free-electrons.com/source/init/main.c?a=arm#L359