Slow File upload and download Speeds via F5

Hello Team.

We are using F5 to load balance two iron port/wsa. when we point proxies directly to the browser we are getting 800Mbps-1000Mbps download speed. But when we point to vip getting only 360-375Mbps. Could you please provide me some idea about this. We are using following config

ltm virtual webproxy {
    address-status yes
    app-service none
    auth none
    auto-lasthop default
    bwc-policy none
    clone-pools none
    cmp-enabled yes
    connection-limit 0
    description proxy-test
    destination 192.168.10.10:squid
    enabled
    fallback-persistence none
    gtm-score 0
    ip-protocol tcp
    last-hop-pool none
    mask 255.255.255.255
    metadata none
    mirror disabled
    mobile-app-tunnel disabled
    nat64 disabled
    partition Common
    persist none
    policies none
    pool Ironport
    profiles {
        XFF-http {
            context all
        }
        tcp {
            context all
        }
    }
    rate-class none
    rate-limit disabled
    rate-limit-dst-mask 0
    rate-limit-mode object
    rate-limit-src-mask 0
    related-rules none
    rules none
    security-log-profiles none
    source 0.0.0.0/0
    source-address-translation {
        pool SNAT_PRD
        type snat
    }
    source-port preserve
    syn-cookie-status not-activated
    traffic-classes none
    translate-address enabled
    translate-port enabled
    vlans {
        VIP
    }
    vlans-enabled
    vs-index 4