Poor SOAP Performance in 7.1

I am having major performance issues calling a web service in 7.1.1 from a .Net client. The IS service runs in 3 seconds, but when called as a SOAP web service (Doc Literal, SOAP 1.1) using the default SOAP processor, it takes 8 - 10 seconds to run.
Does anyone know what could be causing this overhead and is there a good way around it?