function firstScreen() {
    // 收集所有页面的加载时间
    const imgs = document.getElementsByTagName('img')
    const fsItems = []
    const loadEvent = function() {
        // gif避免
        if (this.removeEventListener) {
            this.removeEventListener('load', loadEvent, false)
        }
        const curTime = +new Date()
        fsItems.push({
            img: this,
            time: curTime
        })
    }
    for (let i = 0; i < imgs.length; i++) {
        (function() {
            const img = imgs[i]
            if (img.addEventListener) {
                !img.complete && img.addEventListener('load', loadEvent, false)
            } else if (img.attachEvent) {
                img.attachEvent('onreadystatechange', function() {
                    if (img.readyState == 'complete') {
                        loadEvent.call(img, loadEvent)
                    }
                })
            }
        })()
    }

    // 获取元素在dom中的位置
    function getOffsetTop(elem) {
        let top = 0
        top = window.pageYOffset ? window.pageYOffset : document.documentElement.scrollTop
        top += elem.getBoundingClientRect().top
        return top
    }

    function findMaxTime() {
        const sh = document.documentElement.clientHeight
        let maxTime = 0
        for (var i = 0; i < fsItems.length; i++) {
            var item = fsItems[i]
            var img = item['img']
            var time = item['time']
            var top = getOffsetTop(img)
            if (top > 0 && top < sh) { // 找首屏中的图片
                maxTime = time > maxTime ? time : maxTime
            }
        }
        return maxTime
    }

    window.addEventListener('load', function() {
        const imgTime = findMaxTime()
        const domTime = window.performance.timing.domInteractive // dom完成时间
        let speedTime
        const startTime = window.performance.timing.navigationStart || window.performance.timing.startTime // 页面首页时间
        const screenTime = imgTime > 0 ? imgTime : domTime // 如果没有图片，直接取dom时间
        // eslint-disable-next-line prefer-const
        speedTime = screenTime - startTime
        console.log(speedTime)
    })
}

firstScreen()

直接放到main.js最下方执行即可