1) as the name says, it will force to use only one core
2) can give a boost on some devices and/or some games. I never use it as I don't see a difference on my device
3) It will save yhe game automatically when you quit fpse
4) Can help some games to be compatible, but it's slower, so don't use it with games that work
5) It allows to skip the "bios" page of the PSX, you know, when you start a PSX, you have first the sony logo, etc. With fast boot, you directly go to the game.