如何优化PHP后端功能开发中的页面加载速度？

// 合并CSS文件
$cssFiles = array('style1.css', 'style2.css', 'style3.css');
$mergedCss = '';
foreach ($cssFiles as $file) {
    $mergedCss .= file_get_contents($file);
}
file_put_contents('merged.css', $mergedCss);

// 合并JavaScript文件
$jsFiles = array('script1.js', 'script2.js', 'script3.js');
$mergedJs = '';
foreach ($jsFiles as $file) {
    $mergedJs .= file_get_contents($file);
}
file_put_contents('merged.js', $mergedJs);

<img src="sprites.png" style="background-position: -10px -20px;">

<i class="iconfont icon-user"></i>

$cssContent = file_get_contents('style.css');
$compressedCss = preg_replace('!/*[^*]**+([^/][^*]**+)*/!', '', $cssContent); // 去除注释
$compressedCss = str_replace(array("
", "", "
", "    ", '  ', '    ', '    '), '', $compressedCss); // 去除空白字符
file_put_contents('compressed.css', $compressedCss);

$jsContent = file_get_contents('script.js');
$compressedJs = preg_replace('!/*[^*]**+([^/][^*]**+)*/!', '', $jsContent); // 去除注释
$compressedJs = str_replace(array("
", "", "
", "    ", '  ', '    ', '    '), '', $compressedJs); // 去除空白字符
file_put_contents('compressed.js', $compressedJs);

$cacheFile = 'cachedData.txt';
if (file_exists($cacheFile) && time() - filemtime($cacheFile) < 3600) {
    $cachedData = file_get_contents($cacheFile);
} else {
    $data = queryDatabase(); // 数据库查询
    $cachedData = serialize($data);
    file_put_contents($cacheFile, $cachedData);
}

$cacheFile = 'cachedFragment.html';
if (file_exists($cacheFile) && time() - filemtime($cacheFile) < 3600) {
    $cachedFragment = file_get_contents($cacheFile);
} else {
    ob_start();
    renderFragment(); // 渲染页面片段
    $cachedFragment = ob_get_clean();
    file_put_contents($cacheFile, $cachedFragment);
}

$cacheKey = 'cachedData';
if ($cachedData = getFromCache($cacheKey)) {
    // 使用缓存数据
} else {
    $cachedData = queryDatabase(); // 数据库查询
    saveToCache($cacheKey, $cachedData);
}