This is one of those terms that has been bantered around in the industry over the years and has come to have several subtly different meanings. The original meaning of MIDI delay refers to the time it takes for any active MIDI circuit to handle the signal. Just passing MIDI into, and then directly out of any device (even without doing anything to it) takes some finite amount of time because of the electronics involved in managing and buffering the signal. This is MIDI delay and in most cases it is usually well under 5 ms. The delay is cumulative though. So if you pass your signal through several devices it may be significantly delayed by the time it gets to the last device. Some people also refer to the time it takes an instrument to respond to MIDI commands as MIDI delay. While true MIDI delay is one component of this, there are other factors, such as the speed of the processor in the device. Some instruments react more slowly as they are asked to do more (for example, play more notes at once), but this is technically not MIDI delay. Some musicians claim to be able to hear/feel MIDI delay and do not like performing in situations where MIDI is used. While it’s pointless to dispute what a person says they can perceive, it is important to note that given the speed of sound in air the sound leaving a speaker cabinet on the one side of a 20 foot wide stage would take about 20 ms to reach the ear of a player on the other side.