Complementary Code Keying (CCK) is a modulation scheme used with wireless networks (WLANs) that employ the IEEE 802.11b specification (2.4Ghz).  Modern devices don’t need CCK data rates and using them can lead to spectrum inefficiencies.  When using 2.4Ghz, and you know there won’t be any legacy devices connecting (e.g. some printers, old cell-phones, older laptops etc), then you can improve performance by selecting CCK Disabled on a Wi-Fi radio as shown below.