Each time I start a WebWorks app on my BlackBerry PlayBook I could start crying when something is animated (most of the time, screen transitions). And also if you have a look at webOS, most of the time you know it's only a browser because many apps just take too long to finally start up.
Let's hope Mozilla is doing it better and the whole OS will be more stable and faster than other HTML-OS-experiments. Either that or using really good hardware should help – but as they want to debut in South America, I can't imagine that the devices can or should be too expensive.
I already tested one of the first nightlies some weeks ago and again today and it didn't even run that smooth in an optimized browser window. I know it's still in alpha (or even earlier stage), but there's still a lot work to do. When the first devices are going to be released (and maybe some developer devices given away ;), I'm definitely going to have a closer look and maybe some of my apps are going to be ported!