I'll defrag that for the younger audience - the commodore 64 had a slow external disc drive and best explained "The 1541 was very slow in loading programs because of a poorly implemented serial bus, a legacy of the Commodore VIC-20" https://en.wikipedia.org/wiki/Commodore_64_peripherals#Flopp...
The 1541 maintained compatibility with the VIC. The VIC had very slow firmware routines to communicate with its 1540 disk drive because a shift register in the 6522 VIA chip didn't work properly.
The C64 had an improved VIA with a working shift register. The 1541 attached to a C64 was going to be much faster than the 1540 and the VIC. A board rework in the 1541, however, removed a trace needed for the high-speed operation, so the 1541 and a C64 ended up being no faster than a 1540 and a VIC (and actually slower because the VIC-II in the C64 needed to take control of the bus periodically introducing jitter in the communications).
This is a demo running entirely on the 1541 using a hacked together connector from the drive's serial out to a monitor. Audio is generated using the stepper motor.
I've been trying to find a reference to the board rework removing the high speed serial capability. I heard about it originally in a video or podcast and I'm going to do some digging to find that. To correct the record, though, the board rework story references the C64 motherboard and not the 1541 board.
The 1541 was slow also because of the firmware. There was this kit that I bought sometime in the 80s which, if memory serves, consisted of EPROMs and a small board to be swapped into the C64 and the disk drive in place of their original firmware, then a parallel cable to connect the drive to the C64 via the user port. The firmware also added a much more advanced BASIC interpreter and some utilities. Curiously, by using the parallel cable disk operations became dramatically faster than with the original DIN connector as expected, but if I removed the parallel cable say because I needed the user port, it continued to work, clearly not at the same speed, but still a lot faster than when using the original ROM, so there probably was some room for optimization in the original firmware too. Unfortunately I can't remember the name of that kit, it was one of the many "turbo" this or that on the market at that time, and by today standards looked definitely homemade.
Some of the modern clones of the C64 use JIffyDOS built into the Kernel for faster loading from 1541 drives. The 1541 drive has a 6502 CPU and built in DOS (Commodore DOS not MS-DOS) firmware. It is like getting a whole different computer in the floppy drive which is why it was so expensive.
Believe me, it was very hard to make a slower disk system than the C64 had with the 1541 even back then, and even compared to older hardware. Others in the thread already elaborated why it came to that: A sequence of accidents led to the bus being bitbanged in a terrible way.
