Just got my HP TouchPad, and thought I'd share the Sunspider JavaScript Benchmarks (0.91) for some of the major tablets built-in browsers:
HP TouchPad (32GB), WebOS: 4106ms
Apple iPad 1, iOS 4: 3233ms
Samsung Galaxy Tab, Android: 2077ms
Apple iPad 2, iOS 4: ~2000ms
WebOS's poor performance jibes with what was written here http://randomfoo.net/2011/08/21/hp-touchpad-webos (ie, that its running an older version of Webkit).
If anyone has other tablets, please chime in with how it benchmarks by running it here: http://www.webkit.org/perf/sunspider/sunspider.html