From the kernel configuration help:

CONFIG_IP_ROUTER

Some Linux network drivers use a technique called copy and checksum to optimize host performance. For a machine which is forwarding most packets to another host this is however a loss. This parameter turns off copy and checksum from devices. It may make other changes in the future.

With my setup, I didn't say yes to this setting. Under the heaviest load I can place on my 33.6 modem, the CPU utilization due to networking is around 0.1% as reported by /usr/sbin/statnet.