hey who said the device was supposed to run 15+ applications smoothly, as the memory or the processor get clogged, the device will slow down.

It's up to the users to manage their multi-tasking, what Maemo 5 has given you is a very easy and intuitive way to manage that.

Now in a more scientific way, I'm not sure how we can discover the limitations of the N900 in terms of memory, or processor (and even video).

Are there any benchmarking tools available? Even with such a tool it doesn't answer everything as keeping multiple applications optimized to respond effectively to feedback requires optimization of a different kind, one that cannot be overcome by brute force processing power easily.