I’ve seen a few people complain about longer delays delivering mail alerts in TFS 2010. The reason is that, by default, we batch all notifications and process them every 2 minutes. This was done to accomplish higher scale on very high volume servers. In retrospect a 2 minute delay was not the best default. We will look at changing this for SP1. In the meantime, you can change the default yourself. You can set the delay to 0 if you want, in which case you’ll get the same behavior you saw in TFS 2008. Chris Sidi wrote a post giving some details of how this works.
Also, we are working on a Power Tool that will provide a UI for browsing and modifying these kinds of settings.