Phone getting unresponsive might be an issue of RAM<->SWAP and not really related to core clock rate. Having too many python widgets (or for me any) on your home screens (hildon-home, the stuff you see on your desktops) will leak RAM after a while and start swapping for everything. Your phone should run smooth at 600MHz if freshly restarted (at least 4mins after restart as sometimes thumbnailer and stuff kick in). If your phone runs smooth at any point and is getting unresponsive or slow over time I would bet on swapping.