IEEE 802.11 Wireless Networks have gained popularity, providing users mobility and flexibility in accessing information. Existing solutions for wireless LAN networks have been exposed to security vulnerabilities. Previous study has evaluated the security performance of IEEE 802.11 wireless networks using single server-client architecture. This research investigated the effect of multiple security mechanisms on the performance of multi-client congested and un-congested networks. The effect of different TCP and UDP packet sizes on performance of secure networks was also studied. The results showed that WEP encryption significantly degrades the performance of congested wireless networks. Network performance degradation increased as the number of clients was increased under all security mechanisms.