See https://core.trac.wordpress.org/ticket/58069

README.md

$ composer run-script test 
> phpbench run Bench.php --report=default
PHPBench (1.2.10) running benchmarks... #standwithukraine
with PHP version 7.4.33, xdebug ❌, opcache ❌

\Bench

    benchTrunk..............................I4 - Mo7.625ms (±2.07%)
    benchPatch..............................I4 - Mo3.284ms (±3.08%)

Subjects: 2, Assertions: 0, Failures: 0, Errors: 0
+------+-----------+------------+-----+------+------------+-------------+--------------+----------------+
| iter | benchmark | subject    | set | revs | mem_peak   | time_avg    | comp_z_value | comp_deviation |
+------+-----------+------------+-----+------+------------+-------------+--------------+----------------+
| 0    | Bench     | benchTrunk |     | 100  | 4,242,960b | 7,900.850μs | +1.00σ       | +2.08%         |
| 1    | Bench     | benchTrunk |     | 100  | 4,242,960b | 7,966.200μs | +1.41σ       | +2.92%         |
| 2    | Bench     | benchTrunk |     | 100  | 4,242,960b | 7,639.260μs | -0.63σ       | -1.30%         |
| 3    | Bench     | benchTrunk |     | 100  | 4,242,960b | 7,595.520μs | -0.90σ       | -1.87%         |
| 4    | Bench     | benchTrunk |     | 100  | 4,242,960b | 7,598.570μs | -0.88σ       | -1.83%         |
| 0    | Bench     | benchPatch |     | 100  | 4,242,960b | 3,169.200μs | -1.17σ       | -3.61%         |
| 1    | Bench     | benchPatch |     | 100  | 4,242,960b | 3,184.240μs | -1.02σ       | -3.16%         |
| 2    | Bench     | benchPatch |     | 100  | 4,242,960b | 3,441.970μs | +1.52σ       | +4.68%         |
| 3    | Bench     | benchPatch |     | 100  | 4,242,960b | 3,336.390μs | +0.48σ       | +1.47%         |
| 4    | Bench     | benchPatch |     | 100  | 4,242,960b | 3,308.470μs | +0.20σ       | +0.62%         |
+------+-----------+------------+-----+------+------------+-------------+--------------+----------------+

_wp_normalize_relative_css_links.patch.php

<?php

namespace Patch {

	/**
	 * Makes URLs relative to the WordPress installation.
	 *
	 * @since 5.9.0
	 * @access private
	 *
	 * @param string $css            The CSS to make URLs relative to the WordPress installation.
	 * @param string $stylesheet_url The URL to the stylesheet.
	 *
	 * @return string The CSS with URLs made relative to the WordPress installation.
	 */
	function _wp_normalize_relative_css_links( $css, $stylesheet_url ) {
		return preg_replace_callback(
			'#(url\s*\(\s*[\'"]?\s*)([^\'"\)]+)#',
			static function ( $matches ) use ( $stylesheet_url ) {
				list( , $prefix, $url ) = $matches;

				if ( ! (
					str_starts_with( $url, 'http:' )
					||
					str_starts_with( $url, 'https:' )
					||
					str_starts_with( $url, '//' )
					||
					str_starts_with( $url, '#' )
					||
					str_starts_with( $url, 'data:' )
				) ) {
					// Build the absolute URL.
					$absolute_url = dirname( $stylesheet_url ) . '/' . $url;
					$absolute_url = str_replace( '/./', '/', $absolute_url );

					// Convert to URL related to the site root.
					$url = wp_make_link_relative( $absolute_url );
				}

				return $prefix . $url;
			},
			$css
		);
	}

}

_wp_normalize_relative_css_links.trunk.php

<?php

namespace Trunk {

	/**
	 * Makes URLs relative to the WordPress installation.
	 *
	 * @since 5.9.0
	 * @access private
	 *
	 * @param string $css            The CSS to make URLs relative to the WordPress installation.
	 * @param string $stylesheet_url The URL to the stylesheet.
	 *
	 * @return string The CSS with URLs made relative to the WordPress installation.
	 */
	function _wp_normalize_relative_css_links( $css, $stylesheet_url ) {
		$has_src_results = preg_match_all( '#url\s*\(\s*[\'"]?\s*([^\'"\)]+)#', $css, $src_results );
		if ( $has_src_results ) {
			// Loop through the URLs to find relative ones.
			foreach ( $src_results[1] as $src_index => $src_result ) {
				// Skip if this is an absolute URL.
				if ( 0 === strpos( $src_result, 'http' ) || 0 === strpos( $src_result, '//' ) ) {
					continue;
				}

				// Skip if the URL is an HTML ID.
				if ( str_starts_with( $src_result, '#' ) ) {
					continue;
				}

				// Skip if the URL is a data URI.
				if ( str_starts_with( $src_result, 'data:' ) ) {
					continue;
				}

				// Build the absolute URL.
				$absolute_url = dirname( $stylesheet_url ) . '/' . $src_result;
				$absolute_url = str_replace( '/./', '/', $absolute_url );
				// Convert to URL related to the site root.
				$relative_url = wp_make_link_relative( $absolute_url );

				// Replace the URL in the CSS.
				$css = str_replace(
					$src_results[0][ $src_index ],
					str_replace( $src_result, $relative_url, $src_results[0][ $src_index ] ),
					$css
				);
			}
		}

		return $css;
	}

}

Bench.php

<?php

require_once __DIR__ . '/functions.php';
require_once __DIR__ . '/_wp_normalize_relative_css_links.trunk.php';
require_once __DIR__ . '/_wp_normalize_relative_css_links.patch.php';

/**
 * @BeforeMethods("setUp")
 * @Revs(100)
 * @Iterations(5)
 */
class Bench {

	public $files = [];

	public function setUp() {
		$directory = new RecursiveDirectoryIterator( 'wp-content' );
		$iterator  = new RecursiveIteratorIterator( $directory );

		$this->files = iterator_to_array( new RegexIterator( $iterator, '/^.+\.css$/' ) );
	}

	/**
	 * Test _wp_normalize_relative_css_links() in trunk.
	 */
	function benchTrunk() {
		foreach ( $this->files as $file ) {
			$css = file_get_contents( $file );
			$url = "https://example.com/$file";
			\Trunk\_wp_normalize_relative_css_links( $css, $file );
		}
	}

	/**
	 * Test patched version of _wp_normalize_relative_css_links().
	 */
	function benchPatch() {
		foreach ( $this->files as $file ) {
			$css = file_get_contents( $file );
			$url = "https://example.com/$file";
			\Patch\_wp_normalize_relative_css_links( $css, $file );
		}
	}
}

composer.json

{
    "name": "westonruter/phpbench-wp-normalize-relative-css-links",
    "description": "Benchmarking performance of _wp_normalize_relative_css_links() in WordPress core",
    "repositories":[
        {
            "type":"composer",
            "url":"https://wpackagist.org",
            "only": [
                "wpackagist-plugin/*",
                "wpackagist-theme/*"
            ]
        }
    ],
    "license": "GPL2",
    "authors": [
        {
            "name": "Weston Ruter",
            "email": "westonruter@google.com"
        }
    ],
    "require": {
        "phpbench/phpbench": "^1.2",
        "wpackagist-theme/twentytwentytwo": "^1.4",
        "wpackagist-plugin/gutenberg": "^15.4",
        "wpackagist-theme/twentytwentythree": "^1.1",
        "wpackagist-theme/twentytwentyone": "^1.8",
        "wpackagist-theme/twentytwenty": "^2.2",
        "wpackagist-theme/twentynineteen": "^2.5",
        "wpackagist-theme/twentyseventeen": "^3.2",
        "wpackagist-theme/twentysixteen": "^2.9",
        "wpackagist-theme/twentyfifteen": "^3.4",
        "wpackagist-theme/twentyfourteen": "^3.6",
        "wpackagist-theme/twentythirteen": "^3.8",
        "wpackagist-theme/twentytwelve": "^3.9",
        "wpackagist-theme/twentyeleven": "^4.3",
        "wpackagist-theme/twentyten": "^3.8"
    },
    "scripts": {
        "test": "phpbench run Bench.php --report=default"
    }
}

functions.php

<?php

/**
 * Converts full URL paths to absolute paths.
 *
 * Removes the http or https protocols and the domain. Keeps the path '/' at the
 * beginning, so it isn't a true relative link, but from the web root base.
 *
 * @link https://github.com/WordPress/wordpress-develop/blob/b98b5e59d72a68bc31f99b866fa26f7769d16e97/src/wp-includes/formatting.php#L4717-L4731
 *
 * @since 2.1.0
 * @since 4.1.0 Support was added for relative URLs.
 *
 * @param string $link Full URL path.
 * @return string Absolute path.
 */
function wp_make_link_relative( $link ) {
	return preg_replace( '|^(https?:)?//[^/]+(/?.*)|i', '$2', $link );
}

if ( ! function_exists( 'str_starts_with' ) ) {
	/**
	 * Polyfill for `str_starts_with()` function added in PHP 8.0.
	 *
	 * Performs a case-sensitive check indicating if
	 * the haystack begins with needle.
	 *
	 * @link https://github.com/WordPress/wordpress-develop/blob/b98b5e59d72a68bc31f99b866fa26f7769d16e97/src/wp-includes/compat.php#LL445-L465C2
	 *
	 * @since 5.9.0
	 *
	 * @param string $haystack The string to search in.
	 * @param string $needle   The substring to search for in the `$haystack`.
	 * @return bool True if `$haystack` starts with `$needle`, otherwise false.
	 */
	function str_starts_with( $haystack, $needle ) {
		if ( '' === $needle ) {
			return true;
		}

		return 0 === strpos( $haystack, $needle );
	}
}