It all depends on what software you run under it, certainly Java does not help in this context.
A few days ago I had Debian+lxde running on one of those very small chinese mini netbooks; I can't check now what's the CPU, but it's a 300 MHz one with only 128MB RAM, and the system boots from the SD card (the laptop comes with Win CE). I wouldn't call it snappy for sure, but it's useable.
Not sure why this is being downvoted. I'm afraid you're right that many commercial, closed-source CPU's are faster, but (depending on your application) it's not that bad. CPU's in smartphones are ridiculously overpowered for 95% of the uses cases anyway. Anyway, it's the best 'open' (then documentation is openly available; the CPU is not open source!) processor available, AFAIK.