Interestingly, the "performance" aspect of antivirus on Server 2003 is favorable. The OS is lightweight, and older antivirus engines were built for lower specs. However, this is a moot point. A fast server that is compromised is worse than a slow server that is secure.
If you must keep WS2003 running (e.g., legacy industrial controller, old database), follow this layered approach: antivirus for 2003