I'm not at the layout so I cannot say the following with any real certainty.
Regardless, after initiating PFA you could try swiping right or left on the screen, or tapping the icon at the top of the screen, to go to the Engine Control Screen to change the speed.
Regardless, unless you're running PFA for an engine that moves during the dialog, such as the weed sprayer, there's little to be gained by changing the engine's speed.