Your run of the mill Honda ECU runs at almost 6MHz, so one cycle on that is 166ns, so a few instruction cycles should be all it needs to kick on the extra cylinders, so yeah, maybe a microsecond to deliver ignition. Of course, the injector opens a lot slower than that, so the next firing cycle should see the engine running on all cylinders if the TPS, etc. can track and settle that fast..

I know no one cares, but I'm rambling anyway..