linux isn't this fast because it is semi-monolithic, so it contains a bunch of code to allow a lot of hardware (a lot of it quite dodgy) to work, also over the years it has gathered what is known as cruft, they've been quite sensible about this, but any aging project can't help but acrue some hairy code that cannot be easily replaced without having a big knock-on breakage effect. also remember that linux has to be able to run on crappy old i386 processors and their various derivatives (with their various floating point issues and the fact that they are CISC rather than RISC processors) so it often is optimised for the (s)lowest common denominator.
i would really love to see a healthy modern amiga revival, it'd be cool. and it'd make apple sit up and be a bit more interesting i think.