Comment by cromwellian

The 1541 was slow because the c64’s serial bus was slow. Data was clocked over the bus 1 bit at a time. Various fastloaders sped up the data rate by reusing the clock line itself as a data line (2 bits at a time), later HW adapters adder parallel port or even usb to overcome the serial bus bottleneck.

Basically commodore was gonna use an ieee-488 bus for the drive and then decided it was too expensive late in the design and switched to this hacks serial bus that bottlenecked everything.