The Commodore 1541 floppy disk drive, sold for use with the 64, was notoriously slow for historical and technical reasons:
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 it had to transfer a bit at a time instead of a byte at a time.
Then it 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.
Okay, so given the worst-case combination of those two factors, with no development time allowed for alleviating the problem, one could see how the drive could end up only being able to transfer one bit per horizontal blank = 63 microseconds. 1/(63e-6) = 15873 bits/s = 1984 bytes/s.
But apparently the actual speed was only 400 bytes/s.
Why was the actual speed only a fifth of what would seem to be possible even with the unhappy combination of historical and technical problems?