The difference is that in a modern SP the computer does everything. You saxy were you want to hit with a MRSI and the computer does the rest. I really doubt that even computerized howitzer are as fast as the turret engines of the SPs.
Well in the MRSI mode I agree SP guns are propaply faster but in general aiming and transvere/elevation of the tube, I actually belive that towed APU fitted gun with similar firecontrol like in the 155K98 can do it faster...But then again, I've never operated modern (or unmodern to be clear) SP systems so I cannot be 100% sure.