...VGV's adjusting at almost 40 degrees per second
I would not be surprised if the
actuation system is capable of that slew rate, particularly on a throttle chop where the HP fuel pump is still turning at high speed but flow to the burner is cut way back. There's lots of excess fuel flow available.
But the VGV positioning demand is probably slaved to a core tachometer (either electronic or hydromechanical) - and thus if an acceleration from FI to TOGA takes (e.g.) 5.0 seconds, I would expect the VGV's to make their move in 5.005 seconds. If they moved faster than this, there would be an imbalance in pumping capability from the front of the HPC to the rear, and a stall/surge is risked.
If you ever get the chance to visit a test bench (test cell outside the UK) , you can see this in action. It's really impressive.
