I have to blog about this - as I spent a few days having nightmares about it.
An operation from a websphere service was returning a pdf string, for a payload of about 500KB. The WCF client consuming the service was working fine on the test fixture, but when the operation was integrated in the web solution, with the exact same binding and endpoint, the client was hanging for more than a minute onto the response of this particular operation (every other operation with a smaller payload was OK) before coming back with the deserialized response.
I initially blamed the service, but then noticed (sniffing traffic via fiddler) the response was coming back quick enough and only then the client would hang for more than a minute obviously trying to deserialize or God knows what.
After quite a bit of hacking around on and off, I managed to change the transportmode config setting on the binding from buffered to streamed (I had nothing else left to try!), and it did the trick. In light of this it's pretty obvious that the response was being chunked in parts the size of the buffer and that was probably slowing down the whole process.
Morale of the story: if you've got big payloads transportMode="Streamed" could save your sorry ass.