It's about the rise time and fall time.
- Rise time = time it takes for a signal to go from 10% to 90% of some value
- Fall time = time it takes for a signal to go from 90% to 10% of some value
In the context of transistors, it's the drain/collector node. The schematic setup is just a pull-up resistor to the drain/collector if I'm not mistaken. You properly increase/decrease the voltage at the gate to make the transistor start conducting/stop conducting.
Here's the rise time and fall time for the AIKW50N65DF5:
- Rise time = 12 ns
- Fall time = 6 ns
Here's the rise time and fall time for the AIKW50N60CT:
- Rise time = 29 ns
- Fall time = 29 ns
So by the time the AIKW50N60CT has had its collector go from 0 V to VDD, the collector of AIKW50N65DF5 has been able to go from 0 V to VDD, stay there for 11 ns, and then go back to 0 V.
You won't change the voltage of the gate instantly though, you need to charge the parasitic gate capacitance above the threshold voltage, and in the real world there's only so much current you can force through the gate without damaging it. So if you got a driver IC that can only force 1A into the gate, then with less gate capacitance, you will reach your threshold voltage faster and therefore be able to reach higher frequencies.
Here's the capacitances for the AIKW50N65DF5:
- Input capacitance = 2800 pf
- Output capacitance = 65 pf
- Reverse transfer capacitance = 11 pf
Here's the capacitances for the AIKW50N60CT:
- Input capacitance = 3140 pf
- Output capacitance = 200 pf
- Reverse transfer capacitance = 93 pf
Then there's the gate charge that changes with gate-emitter voltage, in both of the transistors' typical usages, the gate charge for the fast transistor is about 1/3 of the slow one. So in a typical setup, the fast one should reach its threshold voltage about 3 times faster than the slow one.
There're probably many other things that I haven't thought of, but these things alone will be the main focus. I mean the fast transistor's max frequency is 120 kHz, the slow one is 30 kHz. The fast one can be used for a frequency that is 6 times greater than the slow one. The 1/3rd gate charge together with the less-than-half rise time + fall time => 3 × 2 = 6. And the capacitances will just make the fast one about 10% faster, which is probably lost somewhere on the way. So it makes sense.