The 1541 wasn't slow because of the serial bus, the software driving it was deliberately run in a slow cpu intensive mode reputedly because one machine the drive could be used on was bugged and wouldn't run any other way. Drive accelerator cartridges simply loaded new driver code that ran the same serial interface in the faster, uncrippled mode on the C64.
I was a very happy bunny when my accelerator cartridge arrived and the 1541 finally ran faster than my own tape turboloader!
Biting the hand that feeds IT
