Internet Browsing very Slow.

Hi,
We just installed ISA Server 2006 on a 100 client Network with two DC. The primary DC are located in Our Main Server Room and the Secondary DC,ISA Server and the internet Connection (ASDL-1MB)are located on another site using a 2mb lease line connection (Speed is ok).
A user which is Authenticating from the PDC browse the internet fast,But a user from the other site which is authenticating from the Secondary DC browse the internet very slow. But For some reason,if i log onto the pc,using a local account the Browsing of internet works Fine.
Is it a DNS or ISA Problem?