Immigration: My experience here is as a US citizen who's always sitting in the front of the plane. Typically it takes less than five minutes; if multiple planes land at once, maybe 10-15 minutes tops. There's about a five-minute walk from the gate to the immigration hall, depending on which gate you arrive at. YMMV if you are not a US citizen/permanent resident or if you are sitting far back in the aircraft.

Baggage: I rarely check bags at all, but when I have done so, it can take a fair amount of time (up to 30 minutes waiting around in the most annoying cases). PHL--my home airport--is fairly notorious in this regard. This is one key reason that I don't check bags.

Customs: no time at all unless you have duty to pay. You just hand your form to the official on your way out of the baggage hall.