Fuel pump check valve is a prime suspect. It's pretty common for this to go bad, allowing fuel rail pressure to leak down over time. Next cold start, try this: Turn key to ignition on position, and listen for the fuel pump to go on then off. Don't crank until the pump has stopped. You can even cycle it a couple of times for good measure. If it starts better, the problem is the check valve. If everything else is working well, I'd consider replacing the fuel pump as optional, because waiting a few seconds before cranking is much cheaper.