I think your "worst case" is probably a bit too conservative Kai - a 4000 series gate (4049) can sink up to 40mA and source 12mA on a 12 v supply- say less than 10 ohms output impedance, bringing your charging times to nearer 50nSec than 50usec.

Hallo Steve,

I'm not happy with my reply, either. I hate to make such speculations, and may be that my guesswork is annoying some people here. For this I apologize. May be it's better not to give any response, when information is so little?

I understood John in such a way, that he drives with CD4000 inverter (A) a transistor (B) and with this transistor (B) the MOSFET (C). As he stated, that only 0.04mA is drawn from inverter (A), I assumed that transistor (B) gets not enough current to charge gate source capacitance of MOSFET (C) low ohmic enough.
I assumed following: CMOS inverter (A) drives transistor (B) with 0.04mA. Assuming a current gain of about 50, transistor (B) will let flow about 2mA. Related to 12V, with a bad design (which we know nothing about up to now!) this COULD mean the use of a collector resistor of about 12V / 2mA = 6kOhm.
I wanted to focuse, that MOSFETs must be driven very low ohmic, when high speed is wished.

If he would do it the way you stated, he indeed would achieve some much faster switching.

Regards,
Kai