Just randomly off the top of my head and without more information from your side like other network aware applications running on the two systems I would say, Vista is running a Virgin Network stack that has not been beaten into submission by hackers and the IT world in general while OSX is running a battle hardened stack that has been tweaked, polished, and buffed to a high sheen.

But before you make any assumptions, you should check a couple of things for running applications. Namely check what process are on each machine and run "netstat -a" and you will get a list of all network aware applications that are currently active.

When it all comes down to it though, two vastly different platforms with very different concepts of proper use of system resources, and 200ms latency is nothing to be concerned about.