Real-World Performance And Battery Life
Real-World Benchmarks
Early on we discovered how difficult it is to benchmark tablets.
Benchmarking responsiveness with a camera is the fastest approach. Of course, normal cameras won't cut it, since they only shoot at 29 FPS. That's unacceptable if you're trying to measure precise time differences. Going the stop-watch route is no better due to human-introduced errors. That's why I'm using a 1000 FPS high-speed camera to measure performance. Since, 1 frame = 1 millisecond, it’s possible to measure timings with a high degree of accuracy.
The iPad 2 and Xoom employ the same CPU hardware, but the 10 second difference in boot-up time suggests that iOS is a fundamentally leaner operating system. When we look at browser launch times, the Chrome browser in Honeycomb definitely seems more boated than iOS's Safari, since the Xoom under-performs both iPads.
Input lag is the time that passed between pressing a key and text appearing on-screen. This tells you how fast the Xoom is registering an action. Ideally, you want low input lag so that you don't feel the tablet stuttering as you type or click on buttons. The average college student has a reaction time of 200 milliseconds for visual stimuli, so there's no perceptible lag while you're typing with the Xoom.
Battery Life
Testing a tablet’s battery life tends to be highly variable unless you control the entire experience from beginning to end. Cumulatively, touch gestures don’t have a great impact on battery life. The biggest factors are CPU/GPU processing, screen brightness, volume, and Wi-Fi use. In order to accurately measure battery life, I coded a script that automatically plays MP3s at 50% volume while browsing different Wikipedia pages every 12 minutes. This benchmark is probably overkill, but it gives you an idea of a worst-case scenario.
Recharge Time
Charging times are a double-edged sword. Ideally, you want a nice slow charge so that your battery lasts more than a few hundred charges. Fast charge times keep you away from the wall socket longer, but in the long run it cuts down on the health of the battery. Usually, the rate of charge starts to slow down somewhere in the 80% to 95% range, which is why the charging time from 0% to 10% is faster than 90% to 100%.
Wireless Performance
It's wrong to look only at throughput to get a sense of network speed, because there are other factors that affect wireless performance. Latency, processing time, and the type of data being sent also affect the perceived "fastness" of a network connection.
Think of a phone call. Throughput is the audio quality. Latency is the amount of time from when you speak into the phone until the person on the other side hears you. And processing time is the delay as you think of a response to a question and speak into the phone. If we apply this to networking, throughput is the amount of data you can send over time, latency is the lag due to data transmission, while processing time is the overhead incurred by receiving the data.
| Load Web Page (seconds) | CNN | |
|---|---|---|
| iPad | 6.59 | 1.80 |
| iPad 2 | 5.85 | 1.53 |
| Xoom | 5.79 | 0.49 |
Now consider that latency plus processing time equals response time. This is where the Xoom holds an advantage over the iPad 2. Even though Motorola uses the same 802.11a/b/g Wi-Fi + Bluetooth 2.1 controller (Broadcom's BCM4329) as both iPads, the Xoom places a processing priority on network tasks, so it benefits from better response time and better throughput. However, Chrome seems to have a bigger processing overhead, so there's basically no speed advantage in Web page rendering.
