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@geekylou
There's a bunch of things at play that make the C64 floppy drives slower (and more expensive!) than others at the time. The primary limitation is that it's transmitting over a serialized version of a IEEE-488 parallel bus (similar to GP-IB). Nodes on this bus are essentially simple stand alone computers with limited resources. In the case of the C64 this means that not only is there handshaking and framing overhead on any data transmitted on the bus, but the drive can only read from the disk (254 bits at a time) into its own internal buffer OR transmit read data from the buffer over the bus to the computer. It can't do both at the same time (at least not with the stock drive firmware and Kernal routines in the C64s ROM).

[So-called "Fastloader" routines commonly used by games to circumvent the transfer speed issue do so by patching over the drive firmware and Kernal routines with their own custom code that violates the bus standard rendering other nodes that might be on the bus like printers unusable in favour of faster transfers.]

@nina_kali_nina @Havoc