Scroll Check by time.

scrollevent.js

let tempTimeStamp = 0;
        window.addEventListener('scroll', function (event) {
            if(((event["timeStamp"] - tempTimeStamp) / 1000) < 5){ //how many seconds you rely on.
                tempTimeStamp = event["timeStamp"];
            }else{
                console.log(( event["timeStamp"] - tempTimeStamp) / 1000)
                tempTimeStamp = event["timeStamp"];
                //use withinviewport in here.
                
            }
        }, false);