Decrease the time it takes to transition from saturation to cutoff of a 2 terminal phototransistor?
Reduce the load resistance.

For marketing reasons, datasheets typically specify a ridiculously low pull-up/-down resistor of 100 ohms or so.
This alone would reduce the output voltage swing too much, but you can add more components to amplify the signal again, e.g. like this (the load is Q1's base/emitter):