Karl,
One of the problems is that not every MIDI device with a MIDI THRU jack is
built to specs. Some of them are routed through a CPU of some sort and are thus
delayed. It's possible that the opto isolators could be rounding the pulses,
but a schmitt trigger would certainly take care of that.
Are you having a problem with MIDI delays or is this just a rhetorical
question? 8)
David