What bandwidth does "perf-stats" actually measure ?

Good day everyone, 

I am trying to get real-time throughput/bandwidth of the traffic passing the Fortigate stats, but to no avail so far. I need it 30-60 days back, so FortiView does not help here. 

I tried querying perf-stats log column which is sent as System -> Event every 5 minutes by Fortigate to Fortianalyzer. And it indeed shows bandwidth  as Kbits/sec of traffic every 5 minutes BUT ... the values coincide with the WAN interface usage patterns (200 Mbit/sec), while I'd like to have TOTAL traffic passing the Fortigate (specifically for sizing ) at those 5 minute intervals. And there is DMZ <-> Lan traffic that never goes lower 400-500 Mbit/sec, so, for sure perf-stats does NOT measure ALL the traffic. Funny part - on the same Fortigate (80D) which has a hard disk, the daily local reports do show total bandwidth used - up to 1.5-1.6 Gbit/sec.  

My question - may be I miss some configuration to enable start gathering such stats? 

Thanks in advance.

Yuri

N.B. I did search the forum, but all posts I found had in-depth discussions, but no resolution, e.g. https://forum.fortinet.com/FindPost/119261