I have received a response to the Vista network performance issue from Microsoft. Here are some points of interest:
- "We have been looking into this problem and are working on a doc that will go into the technical details of what we have found."
- "Please note that some of what we are seeing is expected behavior, and some of it is not. In certain circumstances Windows Vista will trade off network performance in order to improve multimedia playback. This is by design."
- "The connection between media playback and networking is not immediately obvious. But as you know, the drivers involved in both activities run at extremely high priority. As a result, the network driver can cause media playback to degrade. This shows up to the user as things like popping and crackling during audio playback. Users generally hate this, hence the trade off."
- "In most cases the user does not notice the impact of this as the decrease in network performance is slight. Of course some users, especially ones on Gigabit based networks, are seeing a much greater decrease than is expected and that is clearly a problem that we need to address."
- "Two other things to note. First, we have not seen any cases where a users internet performance would be degraded, in our tests this issue only shows up with local network operations."
- "Second, this trade-off scheme only kicks in on the receive side. Transmit is not affected."
I've been doing some more research into this and I'm coming to the conclusion that the issue is related somehow to Multimedia Class Scheduler service (MMCSS). This is a service that makes sure that multimedia applications have prioritized access to CPU resources. I can't prove my theory because killing MMCSS also disables Windows audio.
As more information is made available, I'll keep you in the loop.