we have couple of CPU values and process Values in the WSA
 In SHD_LOG the CPULd is average of CPU load in 1 minute.
 In SHD_LOG the ProxLD is average of Proxy process load in 1 minute.
 in rate command the Proxy CPU is the Proxy process load on that exact time
 In Status Detail, the CPU load is the amount of exact CPU load on the time when you run the command
you can check the MIBs from - AsyncOS MIB Info for WSA (.txt files) in : Cisco Secure Web Appliance - Cisco
Also this link might be a help : Proactively Monitoring Performance of WSA Using SNMP - Cisco
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