slow speed and sometimes intenet going down for local users

[mustafam]

dear we have fortigate 1500d device we have 7 internet line each line have 100mbps speed. so we configured as a wan llb on this firewall and enable some security filters and also user authentication enable. so what happen here user can not get this speed also sometimes intenet going down for local users but can access via public network.even not popup fortigate authentication page. please check attached images and help me out to solve this issue