If you are using WSE, turn of trace if you are not using it.
<diagnostics><trace enabled="false" input="inputtrace.xml" output="outputtrace.xml"/>diagnostics>
Normally, this is part of my own checklist, but I missed it and it started tacking on up to 3 seconds to each xml web call (albeit the trace file got up to 14 MB). Tsk. Tsk. Fortunately I found it very quickly and it was in a qa environment and not production, but do not make the same mistake.
-Mathew Nolton