Firefox Extension - why does it log page.renderTime -1548207097891 instead of something like = 885

extension-content-script.js

document.onreadystatechange = function () {
    console.log("content script was fired onreadystatechange ");


    class PageMeasurement {
        constructor(url) {

            this.url = url;
            this.date = Date.now();
            var t = window.performance.timing; 
            this.pageLoadTime = (t.loadEventEnd - t.fetchStart);
            this.renderTime =  (t.loadEventEnd - t.responseEnd);	

        }
    }
    

    if (document.readyState === 'complete') {
        
        console.log("document.readyState === complete")
        var page = new PageMeasurement(document.URL);
        console.log("page.renderTime", page.renderTime);	

    }

}

If i paste the class below into the console directly it returns the correct render time. As part of my extension (see above) it does not. Why is this the case?

var p = new PageMeasurement(document.URL)
p.renderTime  // results in 1656

    class PageMeasurement {
        constructor(url) {
            this.url = url;
            this.date = Date.now();
            var t = window.performance.timing; 
            this.pageLoadTime = (t.loadEventEnd - t.fetchStart);
            this.renderTime =  (t.loadEventEnd - t.responseEnd);	
        }
    }