From winsupersite.com:
(yes, it's WIN super site, but the writer is just as critical of MS as anyone out there. One of the few journalists in the field who backs up his opinion or admits when he was wrong)
Performance. Focus group participants were asked to rate the sleep, resume, and shutdown times of the tested PCs. Because of the lab configuration needs, they were not able to test startup times (since restarting the PCs would have reimaged them), so I tested those on each PC set myself. I also examined the number of running services and processes on the PC sets. From a performance standpoint, over 79 percent of participants preferred the Signature PC configuration, though many noted that the differences were minor. In fact, many people wrote in "no preference" even though that wasn't a choice.

In my own more exhaustive tests, six of the seven Signature PC configurations outperformed the OEM PC configurations handily and the seventh was a virtual tie. Overall, Signature PCs outperformed the stock PC configurations. These PCs also had significantly fewer services running at idle when compared to OEM PCs: An average of 55 for Signature PCs vs. 78 for the OEM PCs. (What's missing, of course, is some ongoing performance monitoring solution that will keep these numbers comparatively low.)

Overall preference. At the end of the test, participants were asked to determine which of the PCs they preferred overall. Here, over 95 percent of participants, or all but two, chose the Signature PC configuration. Participants overwhelmingly preferred the Signature configurations and rated these PCs faster, better, and more enjoyable overall. One tester noted they would pay extra for a Signature configuration. Comments about the OEM configurations ranged from negative to unprintable.

There's no doubt that the tested Signature PCs were preferable, overall, to the OEM configurations. Signature PCs are quicker and easier to set up and use, offer cleaner operating environments, and provide better integration with Microsoft's associated products and technologies. Signature PCs perform, on average, far better than OEM configurations, in the initial set up of the machines and in ongoing, power management-related activities. The initial services load on Signature PCs is also considerably lower than that of OEM PCs, which should have a positive long-term impact on overall performance.