Wow that is quite a drop. Are you sure that the reported CPU loads are including total CPU time in all execution modes and not just kernel space (si,hi,sy) as reported by top command? USFW is enabled by default starting in R80.30 regardless of kernel version, so traffic that cannot be fully accelerated by SecureXL is handled by the Firewall workers as fwk processes in process space (us). Needless to say this change will cause a lot more CPU cycles to be expended in user/process space than before and may be skewing the graph.
It is also possible that you have a lot of fragmented traffic in your network, and prior to R80.20 fragmented traffic could not be accelerated at all and would always go F2F/slowpath. That restriction was lifted in R80.20+ due to the extensive changes in SecureXL so that may account for the CPU drop as well.
New 2-day Live "Max Power" Series Course Now Available:
"Gateway Performance Optimization R81.20" at maxpowerfirewalls.com