[CentOS] Performance of CentOS as a NAT gateway

> The other side is a high-end Cisco router managed by our ISP.  Its
> their router statistics that tell us we're peaking at just over 10Mb/s
> coming out of the gateway box.  That was where we first assumed the
> problem must be, so we've been working with them on this problem for
> some while now and have pretty definitely eliminated their equipment
> as the bottleneck.

What is the speed of the link between you and the ISP?

Do they have other customer sites that are set up the same way as yours 
that get significantly better performance?