* Marketing insisted on compatibility with the 1540 (the floppy drive sold with the Vic-20) which was slow because the shift register in the 6522 VIA chip didn't work, so bit shifting had to be done in software with the 6502 instead. Using the 6526 CIA chip instead would have fixed the problem.
* Then transfer had to go even slower because unlike the Vic-20, the 64's video chip has to completely take over the bus one out of every eight active scan lines.
Various solutions, like the Epyx FastLoad cartridge or the JiffyDOS ROM replacement, quickly emerged to fix the problem. They provide 5-10x speedup depending on the solution. DolphinDOS was is even faster as it used uses a parallel cable between the C64 and the 1541 instead of the serial cable, bypassing the bit shifting issue entirely.
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