Well, I'm not an expert but I'm fixing the same thing and have learned a lot in the last month.
The high pressure oil system is designed to provide the oil pressure needed to operate the injectors. Their are sensors that detect when high enough pressure is present to tell the computer to fire the injectors. If the pressure is not held high enough from when it was shut off, the pump has to build it back up. This is a low volume / high pressure pump. That's when the extra cranking comes in.
The possible reason for the oil pressure issues are many. O-rings at the injector. O-rings at the high pressure oil rail. leaking fitting at the HPOP. Dummy plugs and stand-pipes. And a few other less common issues.
These are the issues I'm having. But the FICM and sluggish fuel system will also cause this.
If you don't already have something, Get an electronic monitoring system. I am using a Edge CTS. Set it to watch fuel pressure, FICM voltage, ICM and IPR%. Get it the numbers up before you start the engine and watch all these parameters and it will give you some clues as to what's going on. Then get back on here and talk it out. There are some "Youtube" videos that will help also.