If you look at the physics of it..getting a piston engine from idle (say 4000 RPM) to full power ~ 12,000RPm only has to gain 8000 RPM

A large turnbine will idle around 33,000 RPM and full power is in the neighborhood of 120,000...so you are looking at adding some 90,000 RPM, and no matter how fast you dump fuel in the combustion chamber, it is going to take some finite time to get there...same thing on the way down. The smaller turbines have a range of 120,000 RPM (or more in some cases) difference between idle and full power........