I have fine tuned my fan curves in the BIOS and normally, even when I used to game, the fans wouldn't speed up unless there was actual heat to dissipate. Now, I'm digitizing analog tapes using ffmpeg on Linux with the encoder h.246. This smart little thing auto detects the number of CPU cores available - 18 in my case - and uses them all. As soon as I hit ENTER ("start" or the likes for Windows users), my fans blast at full speed. Not that I'm trying to determine CPU temps by hand feel, but the air coming out of the chassi doesn't feel hot at all - at least compared to when I used to game, but maybe that was more the heat from the GPU? Even so, how could the CPU heat up to temps requiring maximum fan speed in an instant? Or is it so that the fans (or their controller or the likes) anticipate high temps and spin up preemptively?

Hysteresis. Make sure your fan control software offers this. A lot of fan control software is dogshit/designed for older CPUs that didn’t heat up so quickly. You want hyateresis to even out the spikes. It’s essentially a delay in ramping up your fans.

I set my ramp up to be reletively quick, but ramp down to be a lot slower. I think 1 second ramp up, 2 seconds ramp down in my bios. If I had the choice it’d be 2 second ramp up and like 5 seconds for the ramp down.

you didn't mention the cpu model, but i noticed with my amd x3d the core temps rise rapidly and the fans react immediately. heat transfers rather slowly from the core up to the lid and heatsink because of the thick layer of L3 cache. the fans spinning fast doesn't help at all though

The fan curves in your bios are likely tied to CPU temperature, and encoding is going to hit your cpu much harder than gaming will as it’s generally pegging every core at 100%. CPU temperatures can rise pretty quickly especially with an air cooler that doesn’t have as much thermal mass as a water loop. I’d check to see what temperatures it is actually hitting and maybe adjusting your fan curves accordingly. If you have a water cooler, I’d recommend pinning the fan curves to the coolant temp instead if you can as that will give you a more gradual curve.