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My question is multiple:
1. What are the influential spice parameters that optimize the switching speed?
To my knowledge, it's mainly Bf that seems to be the most important for increasing the switching speed, which I vary like this:
.step param X LIST 100 150 200 250 300
.model myModel AKO: 2N2222 NPN (Bf={X})
2. What measurement (equation) can be used to measure the speed?
3. What circuit should I use to compare the switching speed results as a function of the influential parameter?

This is not such a simple question. What are your parameters for switching speed? Presumably rise and fall times? How about delays times? You can still have fast rise and fall times, but with a significant delay time if the transistor is driven into saturation.

I would say that Bf is not one of the most influential parameters for speed. More important are the internal capacitances: Cje and Cjc, and transit times: Tf and Tr. Tr sets the reverse recovery time, i.e. charge storage in the base region on switch off - this is critical in saturated BJT switching circuits.

To assess the speed, you need to .MEAS the rise and fall times at the output, and the turn-on and turn-off delay times referenced to the input source waveform.

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Regards,
Tony