Sorry to hear you are selling your P30. I have a few thousand rounds through a USPc .40 LEM. What you felt before the let off was "stacking". The pressure builds just before the hammer falls. It is more difficult to control the trigger during slow fire unless you have a lot of practice. As some friends of mine and I say this gun/trigger was meant to run fast. It's a combat gun. I'm more accurate with it when I shoot faster.
Gripping my USP (and I've handled the P30 similarly) I have my strong side thumb straight up along the side of the slide and my support side thumb forward and resting on the axis of the slide release lever.