Ah ok, but I remember on Facebook when on my single core 2.93 GHz comp sometimes Opera froze during 10 seconds when I posted something. It was because FB had a shit ton of AJAX running at once. Another example is OmnomIRC in Firefox for some people (although it's much less bad with the new version of OmnomIRC)
Yeah maintaining a ton of active connections can slow things down. If there isn't too much to the ajax, like a simple send some data and receive the reply only once per action then it shouldn't slow down older comps too much, if at all.
I have Omni optimized for a bunch of active connections for OmnomIRC's sake.