Tracking user activity on a webpage (ASP.Net) in an efficient way

Question

In my ASP.Net boilerplate application I want to write an efficient (efficient in terms of performance) javascript code that would allow me to track user activity an a given page including the time at which the user entered the page and the time at which left the page.

To track the user activity I ended up with this code:

function activityWatcher() {
    //The number of seconds that have passed since the user was active.
    var secondsSinceLastActivity = 0;

    //in secs
    var maxInactivity = 30;

    //Setup the setInterval method to run every second. 1000 milliseconds = 1 second.
    setInterval(function () {
        secondsSinceLastActivity++;
        console.log(secondsSinceLastActivity + ' seconds since the user was last active');
        //if the user has been inactive or idle for longer then the seconds specified in maxInactivity
        if (secondsSinceLastActivity > maxInactivity) {
            console.log('User has been inactive for more than ' + maxInactivity + ' seconds');
        }
    }, 1000);

    //The function that will be called whenever a user is active
    function activity() {
        //reset the secondsSinceLastActivity variable back to 0
        secondsSinceLastActivity = 0;
    }

    //An array of DOM events that should be interpreted as user activity.
    var activityEvents = ['mousedown', 'mousemove', 'keydown','scroll', 'touchstart'];

    //add these events to the document & register the activity function as the listener parameter.
    activityEvents.forEach(function (eventName) {
        document.addEventListener(eventName, activity, true);
    });
}

activityWatcher();

This code does the job really well but I have a couple of questions.

-Is this code efficient performance wise or is there a way to make its performance better?

-I'm stuck on how to get the time at which the user entered the page and the time at which the user left the page.

-Lets say I want to save some information (like the time the user entered & left the page etc...) to the database, how to save those info to the db in a performant way?