This is because it doesnt multitask at all, in contrary to what everyone seems to believe. Play quake on it, tab to youtube and quake will stop. Same thing with web browsing. Tab to other app and the webbrowser will pause and stop playing flash content. Its NOT multitasking, its just app-pausing. Some apps are, however, allowing real multitasking such as mp3 playback. you can put 100 windows up, the only thing that will happen is that you will run out of swap-space. Same thing goes to all current phones btw, so its not just this one.
N900 doesnt seem to be able to play youtube fluidly at all.