Improve performance of compute_style_properties method. (#62522)

* Improve performance of `compute_style_properties` method.

Improve logic in compute_style_properties to improve performance. This commit fixes a number of performance issues including.

- Expensive repeated call to `str_starts_with`.
- Return easy if not array.
- Repeated call to `strlen`.
- Move call to `get_property_value` where it is needed.
- Move call to implode to after empty check.

* Fix lint.

* Apply suggestions from code review

Co-authored-by: Joe McGill <801097+joemcgill@users.noreply.github.com>

---------

Co-authored-by: Jonny Harris <jonathan.harris@news.co.uk>
Co-authored-by: Joe McGill <801097+joemcgill@users.noreply.github.com>

lib/class-wp-theme-json-gutenberg.php

@@ -2255,43 +2255,37 @@ protected static function flatten_tree( $tree, $prefix = '', $token = '--' ) {
 	 * @return array  Returns the modified $declarations.
 	 */
 	protected static function compute_style_properties( $styles, $settings = array(), $properties = null, $theme_json = null, $selector = null, $use_root_padding = null ) {
-		if ( null === $properties ) {
-			$properties = static::PROPERTIES_METADATA;
-		}
-
-		$declarations = array();
 		if ( empty( $styles ) ) {
-			return $declarations;
+			return array();
 		}
 
+		if ( null === $properties ) {
+			$properties = static::PROPERTIES_METADATA;
+		}
+		$declarations             = array();
 		$root_variable_duplicates = array();
+		$root_style_length        = strlen( '--wp--style--root--' );
 
 		foreach ( $properties as $css_property => $value_path ) {
-			$value = static::get_property_value( $styles, $value_path, $theme_json );
+			if ( ! is_array( $value_path ) ) {
+				continue;
+			}
 
-			if ( str_starts_with( $css_property, '--wp--style--root--' ) && ( static::ROOT_BLOCK_SELECTOR !== $selector || ! $use_root_padding ) ) {
+			$is_root_style = str_starts_with( $css_property, '--wp--style--root--' );
+			if ( $is_root_style && ( static::ROOT_BLOCK_SELECTOR !== $selector || ! $use_root_padding ) ) {
 				continue;
 			}
+
+			$value = static::get_property_value( $styles, $value_path, $theme_json );
+
 			// Root-level padding styles don't currently support strings with CSS shorthand values.
 			// This may change: https://github.com/WordPress/gutenberg/issues/40132.
 			if ( '--wp--style--root--padding' === $css_property && is_string( $value ) ) {
 				continue;
 			}
 
-			if ( str_starts_with( $css_property, '--wp--style--root--' ) && $use_root_padding ) {
-				$root_variable_duplicates[] = substr( $css_property, strlen( '--wp--style--root--' ) );
-			}
-
-			// Look up protected properties, keyed by value path.
-			// Skip protected properties that are explicitly set to `null`.
-			if ( is_array( $value_path ) ) {
-				$path_string = implode( '.', $value_path );
-				if (
-					isset( static::PROTECTED_PROPERTIES[ $path_string ] ) &&
-					_wp_array_get( $settings, static::PROTECTED_PROPERTIES[ $path_string ], null ) === null
-				) {
-					continue;
-				}
+			if ( $is_root_style && $use_root_padding ) {
+				$root_variable_duplicates[] = substr( $css_property, $root_style_length );
 			}
 
 			// Processes background styles.
@@ -2306,6 +2300,16 @@ protected static function compute_style_properties( $styles, $settings = array()
 				continue;
 			}
 
+			// Look up protected properties, keyed by value path.
+			// Skip protected properties that are explicitly set to `null`.
+			$path_string = implode( '.', $value_path );
+			if (
+				isset( static::PROTECTED_PROPERTIES[ $path_string ] ) &&
+				_wp_array_get( $settings, static::PROTECTED_PROPERTIES[ $path_string ], null ) === null
+			) {
+				continue;
+			}
+
 			// Calculates fluid typography rules where available.
 			if ( 'font-size' === $css_property ) {
 				/*