If all webOS applications are coded in JavaScript (so I've heard; didn't look into it too much), the VM overhead could well be dragging things down. Even V8 is ten times as slow as Java or C#, and v8 isn't even available for ARM.
Interestingly, it would be possible to code native apps on it via browser plugins. Some parts of WebOS do use this it seems, but only to interact directly with the hardware.