The issue is anything that relies on Mach3 to handle an input, is going to have a delay while buffers get emptied.
KFlop is the only interface I know of that can interupt that delay, as it can be programmed so it directly responds to an input change, and not rely on having to notify Mach an input has changed then wait for Mach to decide to do something about the change and add it to yet another buffer.
From what's been mentioned so far, Mach4 will respond far quicker, but I think it will still depend on how individual plugins are programmed.