It's interesting how much of the (latency sensitive) data processing is done on the main CPU; I've not used early Macs much, but given they have a multitasking GUI, would user input be noticeably delayed during disk activity, or could it cause problems to e.g. move the mouse rapidly while it was accessing the disk? 16 CPU cycles per bit sounds like it wouldn't leave much time for anything else.
Early Macs are not really multitasking in the sense that newer machines are. Each running application has to hand back control to the operating system as a part of its main loop (think like video game main loop if you are familiar with that)
In the same way, disk operations themselves are generally blocking and will hand back control to the OS and apps during their read/write cycles so that other things can happen on the computer.
Hard real time was pretty easy to do on the Mac because you could run code in interrupt handlers. For example, dragging the mouse (e.g. to pull down a menu) suspended all processing in the system so that it would be responsive. The floppy driver might have run at an even higher priority, causing the mouse to skip during I/O.
The Macintosh would disable any interrupts that were not level 4 or higher when accessing the floppy disk to prevent data loss. This was basically everything other than the programmers switch. It did however have a check to see if the serial port received data while it was busy with the floppy disk so that could be serviced before a buffer overrun occurs.
