Well whatever thing is displaying CPU use to you is stupid and counting the iowait figure as cpu use. Iowait is cpu being idle. It's different from regular idle in that there would be processes wanting to run, but data they need is not in ram.

While you're in the browser and the browser eats more and more ram, everything you aren't using gets dropped from memory on a least recently used basis. When the browser then has things it needs on emmc and stalls waiting for it, and you try switch to something else, it's not either in ram anymore.

Something very small, that continously or regulary executes the same code, has the best chance of actually continuing to execute smoothly. Not anything huge enough to have a gui though. Under large memory pressure, an app's code to process a button tap will have been dropped from memory in between you tapping on the button, if there's alot of other stuff going on that needs ram.

On N810 I was toying with the idea of creating a small daemon with its memory locked (so it can't be dropped from ram) to monitor memory hogs and pause big users of memory if the system slowed down, letting other smaller tasks complete first. It'd give up attempts at multitasking under heavy memory and IO pressure and instead execute everything as batch mode. Unfortunately my N900 with doubly sized RAM compared to N810 arrived before I got to it..