That's completely inaccurate. Ethereum runs the same piece of code on all these computers in the world. That means a program which is being run on the network when it has 500 computers isn't any faster than a program which is being run on 50 computers.

The benefit you get is this: When 500 computers are running it, then you know you can trust the execution and the 'state' of the virtual machine when it is only being run on 50 computers (or 1 in case of a centralized application).