nickrose:
If you send it 128 MIDI messages back to back, it will process each one in turn, some of them causing display updates or other significant work, and it will not be instant. Obviously it has various tricks to try and speed things up, but there are limits.
Well, I should say that I somehow disagree with your response 
Here is what I've tried:
Select "Band delay" -> "Over the Rainbow". Original settings kept: Mod depth and filter type are mapped to the expression)
With an expression pedal (analog) you can easily obtain a Wah effect, meaning that the H9 is tracking the pedal very well.
With my mp-201 sending CC 15 mapped to expression on the H9, the response from the H9 is extremely jumpy.
The first result tell me that the H9 pedal have plenty enough processing power to follow an analog pedal.
The second result tell me that the conversion from CC received to the signal needed (voltage?) to actually control the H9 is very sluggish.
Well, you actually did the hard part right: Making a stompbox that is responsive enough to process an external signal from an (analog) expression pedal and react fast enough in realtime
But, then I could hardly be convinced that converting a CC to an analog value need much processing. This is all that is needed to make the H9 usable with external midi control.
Midi CC to CV conversion is a well known process, require minimal processing power and a rate of 0.5ms per CC could be achieved with "well known" design. In your case you don't even need the extreme precision required for most CV controlled instruments (regarding the Hz/V curve). This is why I would like your DSP engineers to have a look a it.
LA
