The 1541 was slow because the VIC-20 shipped with faulty hardware that couldn't move data on the serial port at full speed, so they patched their software to ge

The 1541 was slow because the VIC-20 shipped with faulty hardware that couldn't move data on the serial port at full speed, so they patched their software to get it working at a much slower speed. The C64 didn't come with faulty hardware but still retained the same software routines for backwards-compatibility. Those fast loaders then replaced these slow compatible routines with full speed ones.