I switched to Comodo Firewall from Norton on my home PC and from McAfee Enterprise on my work pc - both memory usage and CPU use has decreased tremendously on both computers since I switched and the computers definitely exhibit much less lag when carrying out many tasks.
I have not used ZoneAlarm for several years so not sure how resource usage compares directly but based on my experience versus the two firewalls I mentioned earlier and what I have read on the forums I think you will be very happy with resource usage if you switch to Comodo. The attached shows memory usage for the firewall if you want a direct comparison with your current ZoneAlarm usage.
