If your site got faster on Cogent, then it means your old hosts hardware was not capable of handling your traffic load. Since you were with them for a while, your old P3 at your old host definitely can't compare with your new P4 XEON
What I do know is you left a very rock solid host for a new company. Hopefully they continue to run their operations as smooth as your previous host has, might have been wiser to get a hardware upgrade and stay with the guys you were with.