Cause of high latency in performance SLA in fortigate

Dear All,

Some of the websites are getting monitored from Fortigate performance SLA, getting high latency.

When I check using ping,traceroute in fortigate using CLI showing normal latency.

Even I also checked from LAN machine showing normal latency.

unable to find root cause of it, 

Could you please tell us what could be cause of showing high latency for some websites in Fortigate peformance SLA.

Thanks in advanced.