For example...when I do string.find("..."), will it be faster because the CPU is allocated more?
Very likely. And no - you should not see a difference in ONE string.find method call.
The limiting factor on most virutalization platforms is CPU and memory - you only can have that much memory and that much CPU power in one box. So, you have to keep them "aligned" by basically assuming every slice of memory comes with a slice of CPU. As such, a 2gb server will get a larger slice than a 256mb server. Totally natural.
This is normally / should be done by reservations of some part (distributing like x% of the CPU as reservations) and then giving higher allocation priorities to larger machines.
That said, unless everyone uses up his slice, the excess CPU should be available for all.
Taken from the Rackspace site: