I didn't include it to my previous post due to fact that PCA9865 max output frequency is low (1526Hz).
Ah, yes at such low frequency it's probably not a concern.

IRF3708 applications are high freq DCDC (~3-6MHz switching freq), so there is no need to worry about that parameter too much.
Yes, but not with 10mA gate drive - which is my whole point. The total gate charge for IRF3780 is 24nC so if my math isn't failing me it takes 2.4us to charge/discharge it at a constant 10mA charge rate. Driven by a PCA9865 the current into the gate isn't constant of course so in reality it'll take longer than 2.4us. Trying to switch it at even an low(ish) 20kHz would make it spend a considerable portion of the switching cycle in the linear region. At 1500Hz Gusse is most likely correct and it's not a concern.

/Henrik.