Page "Commodore 1541" Paragraph 19

Initially, Commodore intended to use a hardware shift register ( one component of the 6522 VIA ) to maintain relatively brisk drive speeds with the new serial interface.

However, a hardware bug with this chip prevented the initial design from working as anticipated, and the ROM code was hastily rewritten to handle the entire operation in software.

According to Jim Butterfield, this caused a speed reduction by a factor of five.