[MENTION=11846]Gizmo[/MENTION]! Then its quite obvious. Its a problem for backend.
Now you may be wondering, how come you are getting right speeds on Speed Test. I think its because of the Initial Brust rate, which can shoot up the speeds. For example, if your port is set to 4Mb, then speed test's initial brust will go as high as 400KB/s, but will eventually come down to the actual speeds of your connection.