Yes it is limited to the top speed of the serial and even then there are overheads.
Still wonder what it is that requires full bore speed.
And dont forget any steps in the sketch that need to occur as they all time a finite amount of time too.
12 meg is the max "Theoretical" speed BTW.
USB chipset implementation is the biggest factor along with buffers, bus speeds, cache, etc etc etc.
In practice nothing runs continual at that speed and any I know of that do are only in very short bursts.
Grab yourself a good quality USB drive and measure the transfer speeds you will see what I mean.