I recently had to redesign a cascade buck switcher running at 250kHz that was only 65% efficient, 48V to 15V and then 15V to 5V. The design engineer had used a 50A device for the the low side switching mosfet in the 48V to 15V converter where a 2A device would have been sufficient. The 1.5nF gate capacitance (...)
Likely the other 15 percentage points were in the drain energy (capacitance). Every time that thing switches on, it discharges all ~50V in one big gulp. You can't measure this directly, because the capacitance is in parallel at the transistor -- the current spike doesn't go out the source pin, it stays inside. But the power is there, oh yes it is...
Also, 1.5nF might be the small signal capacitance, but the large signal equivalent is usually triple that, give or take. A better estimate is Qg(tot) / Vg(on). So, 50nC at 10V is 5nF equivalent!
Another good reason to use external gate resistors: keep the driver cool. You usually spec a driver for the current it needs to deliver, no more (because cost, and sometimes because switching loss or quiescent current in the driver itself). Typical drivers are rated by peak current into a quasi-static load (a large capacitor), so a 2A (at 10V) driver is about 5 ohms. To deliver that current into your transistor, you can't add any external resistance.
This power dissipation limits the Qg * Fsw you can run a particular driver at.
You can get a bigger driver and add an external resistor, to get the same total Rg for example, while shifting the heat to a safer and/or cheaper location (resistors). (Whether this is actually cheaper, or more compact, or better than just using a driver with better thermal management, who knows -- there are lots of options.)
Oh, and you can use a quite small driver (a few 100mA, say) paired with a complementary pair of BJTs (like PBSS303NX/PX or any number of other similar types), to get a really low equivalent driver resistance (not quite Rdriver / hFE, but better than a 10x improvement for sure) on the cheap. I should do that from time to time...
Tim