Speed of a mosfet triggered directly from the logic pin of an Attiny85?
Reading the spec sheet it looks like the mosfet should take 3.3nS

MOSFET turn on/off times do not work like that. Simplified, a MOSFET behaves like a large capacitor between gate and source. How quickly (or slowly) that capacitor is charged/discharged relates directly to the on/off switching times.

So for example that 3.3 nS is valid only under the conditions mentioned like output resistance of the source and voltages used. In a simplified form indeed that is an RC calculation.

In your case you should not worry about the delay as it will be significantly smaller than you can detect. This is a small MOSFET for low power applications and an ATtiny MCU can easily drive it. Signals of a few kHz should be no issue.

Also this MOSFET has a very low threshold voltage (about 0.6 V) so even if you use the ATtiny at a low supply voltage (1.8 V or more) it will drive the MOSFET without issue.