Cisco PIX 515E bottlenecks Comcast High Speed Internet

I just switched from a T1 to Comcast HighSpeed Internet 16mb/6mb. I am using a Cisco PIX 515E as our firewall. With the firewall in the works between my laptop and Comcast I get at best 5mb/5mb speeds using Speakeasy speed testing. When I remove the PIX from the equation I consistently get 10mb/5mb. I lose on average between 3-5mb when I put the PIX in the middle. It does not make any sense. I consider configuring the hardware properties of the Outside NIC as far as Duplex and Speed and that didn't make a difference.

I know this is not a bandwidth limitation of the PIX so it has to either be a setting on the PIX or something in the interaction between the Comcast router and the PIX.

Pease help!

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Are you accidently blocking Path MTU Discovery by not explicitly permitting ICMP Fragmentation Needed packets into your network? Such packets could have an IP address of anywhere between the source and destination, so effectively you have to permit "any" for their outside source, and you have to effectively permit as the destination any of your internal machines that can go out to the internet.

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