Thanks very much for the suggestions.
I have no idea why I didn't think of using matrices. I will defenitely do so for the switches, and I'd like to do so for the LED's too. The sharing of data and shift register clock was a great idea, and a great saving of IO.
The clock data is a 5v pulse that indicates to move to the next step in the pattern. I'll have 4 of these for each individual sequencer. The control data consists of 16 faders (for pitch, i.e voltage output), 16 buttons to indicate whether the pitch should be outputted, and 2 lots of 16 buttons to indicate whether to output a pulse. There will also be 4 buttons that are used to select the sequencer that should be modified.
Quick in this case would be <5ms, although for my intended purpose <15ms should be fine, but being a modular synth component, I think that others will find some uses that would leverage a fast update.
John, thanks for the idea. I'm assuming you mean sharing clock and latch? I did not intend to check the inputs as often, yet if I can get both of them down to a reasonable time, then look's like I'll implement this aswell.
Just fixed my post, I meant DAC's instead of ADC's.