Users are trying to determine if any performance issues are caused by their hardware, OS, or applications. Robert Watson spells out five steps to troubleshoot network performance. With decent boxes on 5.3, you should have no trouble maxing out a single gig-e with if_em, assuming all is working well hardware wise and there’s no software problem specific to your configuration. Richard Blum’s book Network Performance Open Source Toolkit, which I read and reviewed last year, also gives good tips.”]
Source: https://taosecurity.blogspot.com/2004/11/great-thread-on-network-performance.html