duplex wcf service data transfer speed in IIS

I have a web service which sends messages back to the client via callback interface. The messages are sent on a different thread. The performance is pretty good when the service is self-hosted. However, when hosted in II5, the messages are sent at slower speed.

For example, on a self-hosted s开发者_开发问答ervice, it takes 0.2s to send a 200KB message. For the IIS hosted service, it takes a whopping 2 seconds. In both cases, the services are using Silverlight 4 Http Polling Duplex binding, transferMode=streamedResponse.

What could be the reasons and what can be done to improve the transfer speed in IIS? I have tried setting aspNetCompatibilityEnabled="true" in web config and the speed got even worse.

http://blogs.msdn.com/b/silverlightws/archive/2010/06/25/http-duplex-improvements-silverlight-4.aspx The article suggest that changing maxOutputDelay can force message to flush.