The Schraeder valve is located on the fuel rail which is what the fuel injectors are connected to. It has a black plastic protective cap that unscrews to expose the valve which is exactly like a tire valve but much bigger. Pushing on the valve's center pin will tell you if fuel pressure is present by raw gas squirting out. Don't do this with a hot engine and place a towel underneath to catch the gasoline.
However, I'm unclear on the exact symptoms you're having. Are you actually having two symptoms? As in 1)The starter turns the engine over normally but it takes 3-4 seconds for the engine to start? And 2) once in a while the starter doesn't engage the engine properly so the starter just spins and makes that whirring sound while the engine doesn't rotate at all?
If you are having those two symptoms I described, here's what might be happening...
1) There is an anti-drainback valve inside the fuel pressure regulator that sits on top of the fuel pump, which sits on/in the gas tank. That valve's job is to keep the fuel rail pressurized with fuel so the engine starts quickly. If it leaks, it allows the fuel in the fuel rail to drain back down into the gas tank. When that happens, it takes several seconds for the fuel pump to get the fuel all the way from the tank back into the fuel rail.
2) The starter solenoid is hanging which means the starter motor isn't pushing its starter gear into the flywheel's geared edge which is what rotates the engine. That can be caused by either a bad starter or a bad/weak/loose/corroded electrical connection at the battery.
Hope that helps.