It’s invoked by JS but the transition time is CSS. The applicable rules are in a minified file, so are hard to find. You might be able to override the rule by adding an equivalent altered rule to the Additional CSS panel of the customizer.

The applicable rules are:

.vc_gitem-animate-slideBottom .vc_gitem-zone-b {
    -webkit-transition: all 1s ease;
    -o-transition: all 1s ease;
    transition: all 1s ease;
}

Change all the current 1s to whatever time interval you want, 0.4s perhaps.