My original thought was the reaction time to produce HHO. But the system would be for cruising where the throttle doesnt vary greatly.

What about a butterfly valve with a storage container that opens in direct relation to the throttle. There would be a reservoir the HHO would go into that would store excess gas that is ready to be sucked into the engine. That way you dont have to worry about the on-demand need as the HHO would already be produced ahead of time.

Of course storing the HHO would be dangerous but if left at a low pressure it would be fine.

So you have the HHO produced in intervals by the cell and contained. You would have to figure out how much HHO is being used based on how much the engine is taking in and create a formula used so that the system knows when to produce more HHO at what amperage.

But Im making it complicated... Could work though.