If you notice when you first start thats why pros split so efficiently to the closest minerals, and also when others comeout they get their own seperate patch.
example
8 mineral patches
your first 8 scvs will not stop running back and forth flawless
however after that you will get that stupid wait when they sit there or run around to find a open one. . if you actually do it right and put the 9th where the first 1 goes... than it works perfect however midgame 50 drones 2 expo :D U REMEMBER THAT HA