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MIDI's serial transmission leads to timing problems.

Experienced musicians can detect time differences of as small as 1 / 3 of a millisecond ( ms ), and a three-byte MIDI message requires nearly 1ms for transmission.

Because MIDI is serial, it can only send one event at a time.

If an event is sent on two channels at once, the event on the higher-numbered channel cannot transmit until the first one is finished, and so is delayed by 1ms.

If an event is sent on all channels at the same time, the highest-numbered channel's transmission will be delayed by as much as 16ms.

This contributed to the rise of MIDI interfaces with multiple in-and out-ports, because timing improves when events are spread between multiple ports as opposed to multiple channels on the same port.

The term " MIDI slop " refers to audible timing errors that result when MIDI transmission is delayed.