How?
If anything wouldn't halving the headways theoretically reduce dwell times since you're boarding half of the amount of passengers? Ignoring spikes of passenger inflow from sources like transferring busses, say every minute a station gets 60 passengers. A train running every 90s will on average intake 90 passengers, meanwhile a train running every 3 minutes will on average in take 180 passengers. If we assume that the former train is 2x smaller, but still has the same entry/exit throughput, nothing should change in terms of dwell times.
Furthermore, we have to consider other factors as well. Often major outliers that increase dwell times would be things like people trying to catch a train that's about to leave in the last second - holding the doors open. The more frequently that a train arrives, the smaller the chance that happens as people will be more likely to not bother to catch the train, and simply just catch the one that's coming in 90s.