SOA is all about build/bye/integrate methodology, It can be applied to huge enterprise applications or with a small application that depends on other apllication to do a particular job.
So if you concentrate more on performance then it 'll be the question regarding the way you choose to implement the SOA.
If you do SOA with, for instance, RESTful web services the performance will be better than if you do SOA with secured SOAP web services and an ESB. That said, there are situations where I would prefer security and reliability over performance. In the end, it all depends on the requirements at hand.
Yes it is totally dependent on the requirement. I work in a fast paced environment interacting with machine at real time. It is hard to even fit Java into such an environment. There is just too much overhead doing web services some times.