diff --git a/.github/workflows/reusable-phpunit-tests-v3.yml b/.github/workflows/reusable-phpunit-tests-v3.yml
index 4f7addcd4e60e..a0d02bb06b00f 100644
--- a/.github/workflows/reusable-phpunit-tests-v3.yml
+++ b/.github/workflows/reusable-phpunit-tests-v3.yml
@@ -269,6 +269,22 @@ jobs:
           TEST_GROUPS: ${{ inputs.phpunit-test-groups }}
           MULTISITE_FLAG: ${{ inputs.multisite && 'multisite' || 'single' }}
 
+      - name: Flag slow PHPUnit tests
+        # Runs even when the test step failed (always()), so the signal still
+        # surfaces on red runs, and never fails the job itself (continue-on-error),
+        # because this is advisory only.
+        if: >-
+          always() && inputs.report && inputs.php == '8.5' &&
+          ( github.event_name == 'pull_request' ||
+            ( github.event_name == 'push' && github.ref == 'refs/heads/trunk' ) )
+        continue-on-error: true
+        run: |
+          if [ -f tests/phpunit/build/logs/junit.xml ]; then
+            php tests/phpunit/prepare-slow-test-annotations.php tests/phpunit/build/logs/junit.xml
+          else
+            echo 'PHPUnit JUnit report not found; skipping slow-test annotations.'
+          fi
+
       - name: Run AJAX tests
         if: ${{ ! inputs.phpunit-test-groups && ! inputs.coverage-report }}
         continue-on-error: ${{ inputs.allow-errors }}
diff --git a/tests/phpunit/prepare-slow-test-annotations.php b/tests/phpunit/prepare-slow-test-annotations.php
new file mode 100644
index 0000000000000..8ed28786b5f24
--- /dev/null
+++ b/tests/phpunit/prepare-slow-test-annotations.php
@@ -0,0 +1,268 @@
+#!/usr/bin/env php
+<?php
+
+/**
+ * Flags slow PHPUnit tests with GitHub Actions annotations and a run summary.
+ *
+ * Usage:
+ *
+ *     php tests/phpunit/prepare-slow-test-annotations.php <junit-file> \
+ *         [threshold-seconds] [max-annotations]
+ *
+ * @package WordPress
+ * @subpackage UnitTests
+ */
+
+/**
+ * Escapes a GitHub Actions workflow command message.
+ *
+ * @param string $value Message to escape.
+ * @return string Escaped message.
+ */
+function wp_phpunit_escape_command_message( $value ) {
+	return str_replace(
+		array( '%', "\r", "\n" ),
+		array( '%25', '%0D', '%0A' ),
+		$value
+	);
+}
+
+/**
+ * Escapes a GitHub Actions workflow command property.
+ *
+ * @param string $value Property value to escape.
+ * @return string Escaped property value.
+ */
+function wp_phpunit_escape_command_property( $value ) {
+	return str_replace(
+		array( ',', ':' ),
+		array( '%2C', '%3A' ),
+		wp_phpunit_escape_command_message( $value )
+	);
+}
+
+/**
+ * Escapes text for a Markdown table cell.
+ *
+ * @param string $value Cell value to escape.
+ * @return string Escaped cell value.
+ */
+function wp_phpunit_escape_markdown_cell( $value ) {
+	return str_replace(
+		array( '|', "\r", "\n" ),
+		array( '\\|', ' ', ' ' ),
+		$value
+	);
+}
+
+/**
+ * Converts a container-absolute test path to a repository-relative one.
+ *
+ * PHPUnit records absolute paths (the repository is mounted at /var/www in the
+ * Docker environment). GitHub annotations need repository-relative paths to
+ * resolve to a line, so a known workspace prefix is stripped when present.
+ *
+ * @param string $file Path recorded in the JUnit report.
+ * @return string Repository-relative path, or the input unchanged.
+ */
+function wp_phpunit_relative_path( $file ) {
+	if ( '' === $file ) {
+		return '';
+	}
+
+	$prefixes  = array( '/var/www/' );
+	$workspace = getenv( 'GITHUB_WORKSPACE' );
+
+	if ( is_string( $workspace ) && '' !== $workspace ) {
+		$prefixes[] = rtrim( $workspace, '/' ) . '/';
+	}
+
+	foreach ( $prefixes as $prefix ) {
+		if ( 0 === strncmp( $file, $prefix, strlen( $prefix ) ) ) {
+			return substr( $file, strlen( $prefix ) );
+		}
+	}
+
+	return $file;
+}
+
+/**
+ * Appends a summary to GitHub Actions or writes it to standard output.
+ *
+ * @param string $summary Markdown summary.
+ * @return void
+ * @throws RuntimeException If the GitHub Actions summary cannot be written.
+ */
+function wp_phpunit_write_summary( $summary ) {
+	$summary_file = getenv( 'GITHUB_STEP_SUMMARY' );
+
+	if ( false === $summary_file || '' === $summary_file ) {
+		echo $summary;
+		return;
+	}
+
+	if ( false === file_put_contents( $summary_file, $summary, FILE_APPEND ) ) {
+		throw new RuntimeException( 'The GitHub Actions step summary could not be written.' );
+	}
+}
+
+if ( $argc < 2 || $argc > 4 ) {
+	fwrite(
+		STDERR,
+		"Usage: php tests/phpunit/prepare-slow-test-annotations.php <junit-file> "
+		. "[threshold-seconds] [max-annotations]\n"
+	);
+	exit( 1 );
+}
+
+try {
+	$file              = $argv[1];
+	$threshold_value   = $argv[2] ?? '1.0';
+	$max_annotations   = $argv[3] ?? '20';
+
+	if ( ! is_numeric( $threshold_value ) || (float) $threshold_value < 0 ) {
+		throw new RuntimeException( 'The slow-test threshold must be a non-negative number.' );
+	}
+
+	if ( ! ctype_digit( $max_annotations ) || (int) $max_annotations < 1 ) {
+		throw new RuntimeException( 'The maximum annotation count must be a positive integer.' );
+	}
+
+	if ( ! is_readable( $file ) ) {
+		throw new RuntimeException( 'The JUnit report could not be read.' );
+	}
+
+	$threshold             = (float) $threshold_value;
+	$max_annotations       = (int) $max_annotations;
+	$reader                = new XMLReader();
+	$previous_libxml_state = libxml_use_internal_errors( true );
+	$reader_is_open        = false;
+
+	libxml_clear_errors();
+
+	try {
+		if ( ! $reader->open( $file, null, LIBXML_NONET | LIBXML_COMPACT ) ) {
+			throw new RuntimeException( 'The JUnit report could not be opened.' );
+		}
+
+		$reader_is_open = true;
+		$slow_tests     = array();
+
+		while ( $reader->read() ) {
+			if ( XMLReader::ELEMENT !== $reader->nodeType || 'testcase' !== $reader->name ) {
+				continue;
+			}
+
+			$time = $reader->getAttribute( 'time' );
+
+			// A testcase without numeric timing (for example a skipped test) carries
+			// no slow-test signal, so it is ignored rather than treated as an error.
+			if ( ! is_numeric( $time ) ) {
+				continue;
+			}
+
+			if ( (float) $time <= $threshold ) {
+				continue;
+			}
+
+			$slow_tests[] = array(
+				'name'         => (string) $reader->getAttribute( 'name' ),
+				'class'        => (string) $reader->getAttribute( 'class' ),
+				'file'         => wp_phpunit_relative_path( (string) $reader->getAttribute( 'file' ) ),
+				'line'         => (string) $reader->getAttribute( 'line' ),
+				'time'         => (float) $time,
+				'time_display' => $time,
+			);
+		}
+
+		$xml_errors = libxml_get_errors();
+	} finally {
+		if ( $reader_is_open ) {
+			$reader->close();
+		}
+
+		libxml_clear_errors();
+		libxml_use_internal_errors( $previous_libxml_state );
+	}
+
+	foreach ( $xml_errors as $xml_error ) {
+		if ( LIBXML_ERR_WARNING < $xml_error->level ) {
+			throw new RuntimeException( 'The JUnit report contains invalid XML.' );
+		}
+	}
+
+	usort(
+		$slow_tests,
+		static function ( $left, $right ) {
+			if ( $left['time'] === $right['time'] ) {
+				return strcmp( $left['class'] . '::' . $left['name'], $right['class'] . '::' . $right['name'] );
+			}
+
+			return $right['time'] <=> $left['time'];
+		}
+	);
+
+	$slow_tests = array_slice( $slow_tests, 0, $max_annotations );
+
+	if ( ! $slow_tests ) {
+		wp_phpunit_write_summary( "No PHPUnit tests exceeded {$threshold_value}s.\n" );
+		exit( 0 );
+	}
+
+	// GitHub Actions renders at most 10 warning annotations per step, so the inline
+	// annotations are capped there while the summary table below can list more.
+	foreach ( array_slice( $slow_tests, 0, 10 ) as $test ) {
+		$properties = array();
+
+		if ( '' !== $test['file'] ) {
+			$properties[] = 'file=' . wp_phpunit_escape_command_property( $test['file'] );
+		}
+
+		if ( '' !== $test['line'] ) {
+			$properties[] = 'line=' . wp_phpunit_escape_command_property( $test['line'] );
+		}
+
+		$properties[] = 'title=' . wp_phpunit_escape_command_property( 'Slow PHPUnit test' );
+		$message      = sprintf(
+			'%s::%s took %ss',
+			$test['class'],
+			$test['name'],
+			$test['time_display']
+		);
+
+		printf(
+			"::warning %s::%s\n",
+			implode( ',', $properties ),
+			wp_phpunit_escape_command_message( $message )
+		);
+	}
+
+	$summary = "### Slowest PHPUnit tests (main suite, over {$threshold_value}s)\n\n";
+	$summary .= "| Test | Time (s) | File:line |\n";
+	$summary .= "| --- | ---: | --- |\n";
+
+	foreach ( $slow_tests as $test ) {
+		$location = $test['file'];
+
+		if ( '' !== $test['line'] ) {
+			$location .= ( '' !== $location ? ':' : 'Line ' ) . $test['line'];
+		}
+
+		if ( '' === $location ) {
+			$location = '&mdash;';
+		}
+
+		$summary .= sprintf(
+			"| %s::%s | %s | %s |\n",
+			wp_phpunit_escape_markdown_cell( $test['class'] ),
+			wp_phpunit_escape_markdown_cell( $test['name'] ),
+			$test['time_display'],
+			wp_phpunit_escape_markdown_cell( $location )
+		);
+	}
+
+	wp_phpunit_write_summary( $summary );
+} catch ( Throwable $error ) {
+	fwrite( STDERR, $error->getMessage() . "\n" );
+	exit( 1 );
+}