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I have a BeWAN iBox A5100 wireless router and a 100 Mbps direct internet connection (non-DSL). When I use the bridged IP option, speedtest.net reports downlink speed as 85mbps which is quite OK.
When I switch to NAT, the speed drops to 35mbps. The computer where I run the test is connected over 150 mbps WLAN and the rest of the setup is exactly the same. There are no other devices connected to the router. I tried switching between NAT and bridged IP a few times and the result is always the same. The problem with bridged IP is that my ISP provides a max of 4 public IPs which is not always enough for all connected devices.
My question is, is it some sort of known behavior that NAT is always slower, is it a problem with the router, or something else?
Sounds like your router's CPU is a little underpowered for its task, maybe you can test another (newer) router? A cheap alternative would be a Raspberry Pi that handles the routing/natting, it runs fast and reliably while consuming very little power. – Robert – 2013-12-04T12:51:28.670