The results below are from our combined Battery Eater Pro (worst case) and Web Browsing only (almost best case) tests. BEP beats on the CPU, GPU, disk and memory while it renders a 3D image and rotates it in real time on the screen. Our light duty, web browser test refreshes a web page of mixed text, graphics, HTML and Flash, every 3 minutes. Both tests are run with display brightness set to 50% with no sleep timers. All other power plan options are left as delivered from the manufacturer. Please note we currently only have two ultrabook datapoints listed here for the web browser only test.
Regardless, it appears that Dell more aggressively Turbos up the CPU under workloads but also has Windows power plan settings configured to dial things back hard when idle or with light loads. This would explain why the XPS 13, with a slightly slower CPU than the Zenbook, beat all others by a small margin in our performance benchmarks, but also has the worst battery life under load and the best battery life so far when idle or under light loads (just browsing the web).
In short, this probably is what most end users would want. The XPS 13 cranks up quick to crunch workloads on demand but then pulls way back down when idle or when workloads are light, in order to maximize battery life.
Incidentally, we looped a 1080p H.264 video clip continuously on a full charge and the XPS 13 lasted 4 hours almost to the minute, again at 50% display brightness. Not bad at all.