Well just now was the first time that I have experienced this in Iron, since a few days ago after cleaning out the cache. I had just installed Windows updates (and chose not to reboot), and I looked in Windows' task manager and saw well under 100 megs of RAM that were "free" (whatever that's supposed to mean). Oh also, I had just separated two tabs away from each other, giving each their own window.
So perhaps it is a matter of RAM and how hard the browser has to work (assuming that a full cache would make it work a bit harder...). I rebooted and did all this stuff, and the browser was fine.
I take it that there aren't any shortcuts to tell Iron to clean the cache--that running Iron Cleaner is the quickest way?