only sometimes after. it only happens in safari after i've been using it awhile. maybe it has to do with a shortage of ram or maybe the iphone's doing one too many things at once? i don't know. try restarting it, and see if it speeds things up.
Restarting it with holding down the sleep/wake button and home key works for me to fix this issue. Or when its actually happening just hold the Home key down for 6-8 seconds and it will force quit.