用img和用script發(fā)送get請求，后者貌似要稍微快一點，這是為什么？

//todo function base64Encode
let d = {};

setTimeout(function(){
                let count = 0;
                let time = 0;
                let loadedCount = 0;
                (function send() {
                    if (count > 10) {
                        return;
                    }
                    let img = document.createElement('img');
                    let t = +(new Date);
                    img.src = 'http://asdf.com/?d=' + base64Encode(JSON.stringify(d)) + '&t=1&r=' + Math
                        .random();
                    img.onload = img.onerror = () => {
                        let diff = +(new Date) - t;
                        time += diff;
                        console.log('img',time,count,time/count);
                    }
                    document.body.appendChild(img);
                    count++;
                    send();
                })();
            },0);

            

            setTimeout(function () {
                let count = 0;
                let time = 0;
                let loadedCount = 0;

                function send() {
                    if (count > 10) {
                        return;
                    }
                    let script = document.createElement('script');
                    let t = +(new Date);
                    script.type='text/javascript';
                    script.src = 'http://asdf.com/?d=' + base64Encode(JSON.stringify(d)) +
                        '&t=1&r=' +
                        Math.random();
                    script.onload = script.onerror = (e) => {
                        let diff = +(new Date) - t;
                        time += diff;
                        console.log('script',time,count,time/count);
                    }
                    document.body.appendChild(script);
                    count++;
                    send();

                };
                send();
            }, 200);

以上代碼分別用img和script向某地址發(fā)送get請求。頁面刷新多次，90%的時候得到的結(jié)果都是script比img要快一些，個人估計用時要少20%左右。

如圖

這是為啥？