@Swoop
The vcore is the actual voltage that the motherboard supplies to your processor.
My experiences from when I upgraded my G3 to a G4 a few years ago suggest that too low a vcore will cause crashes.
It will not slow down your system, just stop it working all together if it is wrong. Be warned though, too high a vcore can trash your cpu!!!
You could read through this
http://amigaworld.net/modules/newbb/v ... hp?topic_id=6180&forum=14thread over at AW - for some useful info.