Anti-Virus Software, Firewalls, Pop-up Blockers, Proxies, etc.
In addition to the issues previously mentioned regarding scheduled tasks and scanning software, there are some additional issues to consider regarding anti-Virus software, firewalls, pop-up blockers, etc. First is that viruses, spy ware, etc. can drastically affect the performance and even stability of your systems and data, and therefore it is very important that you protect your servers with virus protection software, firewalls, etc. The considerations listed previously should in no way be considered as a recommendation not to use such security measures.
However, there are many different varieties of such security software, and some of them may interfere with legitimate network traffic or server functionality. Usually, such interference results in outright runtime errors rather than causing poor performance. However, depending on the implementation and configuration of the specific software or security system used, either performance issues and/or failure of important runtime functionality could potentially occur.