SOAP adds overhead so yeah, a RESTlet would be faster in theory but how much faster you'd have to test it out. NetSuite doesn't publish performance metrics and they get pissy if you do it. Its way easier to use REST APIs for sure. In general if I see a SOAP integration I run away. There's no reason to have a SOAP integration these days unless its legacy and you just dont want to bring it into this century or you have no control over it.