FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
This article describes the use of the performance statistics logs and how to adjust their frequency or disable it when needed.

FortiOS has a feature that creates a periodic log entry with the general information about the performance of the unit.
It includes information about the CPU and memory usage, together with the concurrent sessions count and setup-rate. Please see the sample below:

itime=2020-09-01 11:13:06 vd=root bandwidth=13/16 totalsession=53 disk=1 dstepid=3 devid=FGVM04TM20001209 disklograte=0 logdesc=System performance statistics msg=Performance statistics: average CPU: 0, memory: 48, concurrent sessions: 53, setup-rate: 0 idseq=267453240667275345 type=event eventtime=1598951586 mem=48 dtime=2020-09-01 11:13:06 devname=FGT-VM-HEL setuprate=0 dsteuid=0 itime_t=1598951586 euid=3 fazlograte=20 date=2020-09-01 level=notice epid=3 logid=0100040704 subtype=system time=11:13:06 action=perf-stats cpu=0
This regular report can be very useful when troubleshooting changes on the behavior of the units in terms of the resource usage because it provides records to track those changes over time.
However, in some situations this feature is unwanted because of the frequency of these reports so there is a configuration parameter to be modified in those cases.

The option is available just from CLI and it is as follows:
# config system global
    set sys-perf-log-interval <0-15>
0     <----- Disabled
1-15   <----- Time in minutes between logging. 5 is default value.
Performance statistics are not logged to disk. It is showing on memory. Performance statistics can be received by a syslog server or by FortiAnalyzer.

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Technical Note: No system performance statistics logs