For RAM: /proc/slabinfo gives you a lot of memory allocation info. The second column is the 'number of objects' and the third column is the 'size of object' For CPU: the 'system' on the CPU states field handles the cpu load on the system. For disk: there is not a consideration. Netfilter shouldn't be affected by disk I/O. The only serious slowdowns I had with Netfilter were when I had a memory leak. I had 2.4.20 redhat derived kernel with a semi-recent patch-o-matic. The memory kept on building up, and eventually I was so short on memory that the whole system started grinding doing anything. I updated to 2.4.22 and the problem has totally disappeared, as well as those annoying slowdowns.
