That's a pretty broad question. The proximity to each other is not the question, decoupling caps want to be as close as possible to the pin they are decoupling. If the pins are far apart, then the caps will be far apart, if the pins are close together, the caps will be close together.
What do you mean "How do they affect the overall performance"? They could have negligible effect, or they could make the difference between your circuit working or not working. The question is meaningless.
I suspect you need to educate yourself about the whole concept of decoupling- why you do it, how to do it, etc.
Decoupling caps perform several functions, which may vary from circuit to circuit. They can prevent local transients from propagating to the rest of the system by providing a "local" power source. They can protect a device from noise from other parts of the circuit. Etc.