The designers planned on using a shift register in the 6522 VIA chips to implement fast serial loading, but an undocumented bug in that chip forced them to fall back to the slow bit banging version that shipped
it's fun seeing c64 people on the defensive about it, a nice change from getting lectures from them about how their graphics were the pinnacle of 8-bit computing
The Apple II disk drives, on the other hand, were not only cheap (Apple was different then!) and fast, but were powered by the ribbon cable connecting them to the computer.
- C64 shipped with 6526, fixed version of 6522 without shift register bug
- C64 is incompatible with 1540 anyway
They crippled C64 and its floppies _for no reason_.
The C64 data speed actually ended up being even slower than the VIC-20. You can read the full detailed here: https://imrannazar.com/articles/commodore-1541
They finally got it right in the C128 though!