Because the key schedule
is inherently sequential, even if dramatically more
hardware were thrown at the problem (e.g., two Sboxes for the key scheduler), the key schedule speed
could not be significantly increased relative to the
speed of the encryption itself.
For RC6, if a separate key schedule pipeline stage
is used, the additional logic required is non-trivial
(e.g., another 32-bit variable rotator or two), increasing cost. Also, the number of clocks required
to run the full key schedule is about 180, assuming