router performance

i have set up a router in linux2.4(RedHat version 9) as follows:-
1)disable ip forwarding
2) sniff packets using pcap
3)check for my filter specifications
4)if packet is allowable,write the packet into the outgoing interface
using raw sockets.
note:-my filter specifications cannot be met out by using iptables.
the performance of the system declines owing to buffer
overflows,please help by any suggestion on performance enhancement of
my system.
Thanks in Advance