So, whenever you see an build with afterburner engaged and it goes from 100 MPH to 700 MPH in ten seconds, you might say

"Well that's not realistic!"

But here's the thing, it is, you see, planes with powerful engines naturally have fast acceleration, SP has an physics engine, the F14 for example, could go from 150 to 610 Knots in less then 10 seconds. So don't worry if you have an plane that goes mach 2 in half a second, if your engine is that powerful, then it's that powerful! But if you want an gradual responds use,
throttleResponse in the input section, and set it to 0.04 or anything you want.