Hard to tell you what is going on without much more info.
How much bandwidth do you use in a month, what was the hardware specs of both machines, what scripts are running, is apache/mysql properly tuned, etc. etc.
at less then 1500 gb per month, I dont think you'd notice traffic slow down on a 10mbps line. Unless of course you are on an oversold service, with 50 x 10mbps servers all hooked up to a single 100mbps uplink
