diff --git a/inc/attachment_cache.php b/inc/attachment_cache.php
index 64fa4a62..392caa9c 100644
--- a/inc/attachment_cache.php
+++ b/inc/attachment_cache.php
@@ -5,6 +5,13 @@
  */
 class Optml_Attachment_Cache {
 	const CACHE_GROUP = 'om_att';
+	/**
+	 * Most URLs kept in the in-memory map during one request.
+	 *
+	 * A page with many distinct image URLs would otherwise grow the map for the whole request.
+	 * When it is full, the older half is dropped; those URLs are still in the persistent cache.
+	 */
+	const MAX_MEMORY_ENTRIES = 2000;
 	/**
 	 * Local cache map.
 	 *
@@ -32,10 +39,10 @@ public static function get_cached_attachment_id( $url ) {
 			return self::$cache_map[ $cache_key ];
 		}
 
-		$value                         = wp_using_ext_object_cache()
+		$value = wp_using_ext_object_cache()
 			? wp_cache_get( $cache_key, self::CACHE_GROUP )
 			: get_transient( self::CACHE_GROUP . $cache_key );
-		self::$cache_map[ $cache_key ] = $value;
+		self::remember( $cache_key, $value );
 
 		return $value;
 	}
@@ -51,7 +58,7 @@ public static function get_cached_attachment_id( $url ) {
 	public static function set_cached_attachment_id( $url, $id ) {
 		$cache_key = self::get_cache_key( $url );
 		// We cache also in memory to avoid calling DB every time when not using Object Cache.
-		self::$cache_map[ $cache_key ] = $id;
+		self::remember( $cache_key, $id );
 		// If the ID is not found we cache for 10 minutes, otherwise for a week.
 		// We try to reduce the cache time when is not found to
 		// avoid caching for situation when this might be temporary.
@@ -61,6 +68,21 @@ public static function set_cached_attachment_id( $url, $id ) {
 			: set_transient( self::CACHE_GROUP . $cache_key, $id, $expiration );
 	}
 
+	/**
+	 * Keep a value in the in-memory map, dropping the older half when it is full.
+	 *
+	 * @param string                   $cache_key The cache key.
+	 * @param int|numeric-string|false $value The attachment ID (a numeric string when read back from a transient), or false when it is not cached.
+	 *
+	 * @return void
+	 */
+	private static function remember( $cache_key, $value ) {
+		if ( ! isset( self::$cache_map[ $cache_key ] ) && count( self::$cache_map ) >= self::MAX_MEMORY_ENTRIES ) {
+			self::$cache_map = array_slice( self::$cache_map, (int) ( self::MAX_MEMORY_ENTRIES / 2 ), null, true );
+		}
+		self::$cache_map[ $cache_key ] = $value;
+	}
+
 	/**
 	 * Generate cache key for URL.
 	 *
diff --git a/tests/test-attachment-cache.php b/tests/test-attachment-cache.php
new file mode 100644
index 00000000..67ce1f29
--- /dev/null
+++ b/tests/test-attachment-cache.php
@@ -0,0 +1,82 @@
+<?php
+/**
+ * Attachment cache tests.
+ *
+ * @package     Optimole-WP
+ * @subpackage  Tests
+ */
+
+/**
+ * Class Test_Attachment_Cache.
+ */
+class Test_Attachment_Cache extends WP_UnitTestCase {
+
+	public function setUp(): void {
+		parent::setUp();
+		Optml_Attachment_Cache::reset();
+	}
+
+	public function tearDown(): void {
+		Optml_Attachment_Cache::reset();
+		parent::tearDown();
+	}
+
+	/**
+	 * Read the in-memory map.
+	 *
+	 * @return array<string, int|numeric-string|false>
+	 */
+	private function memory_map() {
+		$property = new ReflectionProperty( Optml_Attachment_Cache::class, 'cache_map' );
+		$property->setAccessible( true );
+
+		return $property->getValue();
+	}
+
+	/**
+	 * A page with many distinct uncached image URLs must not grow the in-memory map for the whole request.
+	 */
+	public function test_memory_map_is_bounded_on_cache_misses() {
+		$limit = Optml_Attachment_Cache::MAX_MEMORY_ENTRIES;
+
+		for ( $i = 0; $i <= $limit; $i++ ) {
+			Optml_Attachment_Cache::get_cached_attachment_id( 'https://example.org/wp-content/uploads/miss-' . $i . '.jpg' );
+		}
+
+		$this->assertCount( (int) ( $limit / 2 ) + 1, $this->memory_map() );
+	}
+
+	/**
+	 * The newest URLs stay in memory when the older half is dropped, and the dropped ones are
+	 * still found in the persistent cache.
+	 */
+	public function test_dropped_entries_are_still_found_in_the_persistent_cache() {
+		$limit = Optml_Attachment_Cache::MAX_MEMORY_ENTRIES;
+
+		for ( $i = 0; $i <= $limit; $i++ ) {
+			Optml_Attachment_Cache::set_cached_attachment_id( 'https://example.org/wp-content/uploads/image-' . $i . '.jpg', $i + 1 );
+		}
+
+		$map = $this->memory_map();
+		$this->assertCount( (int) ( $limit / 2 ) + 1, $map );
+		$this->assertContains( $limit + 1, $map );
+		$this->assertNotContains( 1, $map );
+
+		$this->assertSame( 1, (int) Optml_Attachment_Cache::get_cached_attachment_id( 'https://example.org/wp-content/uploads/image-0.jpg' ) );
+	}
+
+	/**
+	 * Updating a URL that is already in memory never drops other entries.
+	 */
+	public function test_updating_a_known_url_keeps_the_map() {
+		$limit = Optml_Attachment_Cache::MAX_MEMORY_ENTRIES;
+
+		for ( $i = 0; $i < $limit; $i++ ) {
+			Optml_Attachment_Cache::set_cached_attachment_id( 'https://example.org/wp-content/uploads/image-' . $i . '.jpg', $i + 1 );
+		}
+		Optml_Attachment_Cache::set_cached_attachment_id( 'https://example.org/wp-content/uploads/image-0.jpg', 99 );
+
+		$this->assertCount( $limit, $this->memory_map() );
+		$this->assertSame( 99, Optml_Attachment_Cache::get_cached_attachment_id( 'https://example.org/wp-content/uploads/image-0.jpg' ) );
+	}
+}