The only thing I might think to try is rather then using BizTalk HTTP Adapter for your call try using a .net component call inside an Orchestration.
That would reduce all the Adapter and Message Box overhead. Just make sure you know the cost of this in terms of loss of message tracking, batching, and potential reliability.
I don’t know how this would perform when compared to the adapter but I would be curios to get your thoughts on this and find out how it works.
I would think it would be faster but not scale as well.