Fortigate 100A & Dual WAN

Hello,

We have the Fortigate 100A and we have connected  2 ISP to WAN ports. 

config system interface
    edit "wan1"
        set vdom "root"
        set ip X.X.X.X 255.255.255.0
        set allowaccess ping
        set gwdetect enable
        set detectserver "8.8.8.8"
        set log enable
        set type physical
        set alias "ISP1"
    next
    edit "wan2"
        set vdom "root"
        set ip X.X.X.X 255.255.255.224
        set allowaccess ping
        set type physical
        set alias "ISP2"

We have Static route:

config router static
    edit 1
        set device "wan1"
        set gateway I.S.P.1
    next
    edit 2
        set device "wan2"
        set gateway I.S.P.2
        set priority 100

How to do that the slow internet from ISP1 (ping> 800ms), it has switched the default gateway to ISP2 ?