Not to mention, a LOT of the early 2000's chevy's had a long start when they got up in age/mileage. My buddy had a 2003 with the same set up and his was almost comical how long it would take to start, usually 2-3 "normal" cranks to start. But also turning the key on and letting the fuel pump prime for a second would help it to start faster.
I do agree with what you are suspecting. It does sound like fuel, weather that be the pump not having enough pressure, the fuel filter / injectors being partially clogged.
If it were the FPR however, it would make for a more consistent issue even at idle. The regulator is meant to limit the pressure from the pump, so if that were out, It would run pig rich even at idle, and would hesitate severely when you gunned it (may even stall).
I would check the timing too, with you changing the cap and such it may have thrown off the timing enough to mess with it under load, not likely but doesn't hurt to check it off the list just in case.