Network World put out a great article on router performance when filters are applied to an interface. The results were that most of the major router vendors they don't have a problem with it and can handle it at least on T1 speeds without adding much latency and without reducing throughput. But, really who would have a problem with just a single T1 with out without packet filters? Cisco. And this test wasen't done with their smallest router, it was done with the 2651. I admit, the 2651 is getting up there in age but it should be able to handle a T1 no matter what, it used to be the top of the line for the 2600 series (now there is a 2600XM and a 2691). Make you wonder as we have all been taught to put access lists on our edge routers to filter out the RFC 1918 incoming as well as our own IP space and more depending on whether you are also using it as a firewall. Check out the article , the degredation looks pretty severe from the charts.