Index: src/wp-includes/class-wp-user.php
===================================================================
--- src/wp-includes/class-wp-user.php	(revision 44784)
+++ src/wp-includes/class-wp-user.php	(working copy)
@@ -233,6 +233,12 @@
 		}
 
 		if ( false !== $user_id ) {
+			// Prevent non-existent users from triggering multiple queries
+			$notusers = wp_cache_get( 'notusers', 'users' );
+			if ( isset( $notusers[ $user_id ] ) ) {
+				return false;
+			}
+
 			if ( $user = wp_cache_get( $user_id, 'users' ) ) {
 				return $user;
 			}
@@ -244,6 +250,14 @@
 				$value
 			)
 		) ) {
+			// User does not exist, so we must cache its non-existence
+			if ( ! is_array( $notusers ) ) {
+				$notusers = array();
+			}
+
+			$notusers[ $user_id ] = true;
+			wp_cache_set( 'notusers', $notusers, 'users' );
+			
 			return false;
 		}