MySQL Poor performance INSERT or PHP PDO

Question

Usually when a user comes to my website he 'subscribes' to around an average of ~50 events at once, which we save in a mapping_table that links many users to many events. maping_table has around 50k entries, growing by several hundreds each day.

INSERT ing those ~50 lines in the mapping table takes around 10 to 20 seconds which makes my website appear very slow.

I reach a point where I can't figure out how to improve the following PHP or SQL in order to lower the INSERT time as it seems pretty basic.

Here is the SHOW CREATE statement :

    CREATE TABLE `mapping_table` (
     `user_id` char(21) NOT NULL,
     `event_id` char(21) NOT NULL,
     `update_id` char(10) NOT NULL,
     PRIMARY KEY (`user_id`,`event_id`),
     KEY `event` (`event_id`),
     CONSTRAINT `mapping_table_ibfk_1` FOREIGN KEY (`user_id`) REFERENCES `users_table` (`user_id`) ON DELETE CASCADE ON UPDATE NO ACTION,
     CONSTRAINT `mapping_table_ibfk_2` FOREIGN KEY (`event_id`) REFERENCES `events_table` (`event_id`) ON DELETE CASCADE ON UPDATE NO ACTION
    ) ENGINE=InnoDB DEFAULT CHARSET=latin1

My php script for the insert part :

    $update_id = 'RandomString';
    $conn = new PDO($pdo_serv, $db_user, $db_pwd);
    $stmt = $conn->prepare("INSERT INTO `mapping_table` (`user_id`, `event_id`, `update_id`)
                    VALUES (:uid,:eid,:upd)");

    //GO THROUGH EACH USER SUBSCRIBED EVENTS AND INSERT THEM
    foreach($subscribed_events AS $event) {
        $stmt->bindParam(':uid', $userid);
        $stmt->bindParam(':eid', $event->id);
        $stmt->bindParam(':upd', $update_id);
        $stmt->execute();
    }

I'm open to any suggestions.

Additional information :

If I microtime each insert I get an average of 300ms to 1sec per insert.

I have tried deleting the UNIQUE KEY , then all FOREIGN KEYS , then all INDEXES but those did not make any difference in performace.

Answer 1

Following comments on my questions I modified my php as follow for batch INSERT :

$update_id = 'RandomString';
$conn = new PDO($pdo_serv, $db_user, $db_pwd);

$sql = "INSERT INTO `mapping_table` (`user_id`, `event_id`, `update_id`)
                VALUES ";
$values = "";

//GO THROUGH EACH USER SUBSCRIBED EVENTS GENERATE VALUES STRING
foreach($subscribed_events AS $event) {
    $values .= "('".$userid."','".$event->id."','".$update_id."'),"
}

//Proceed batch insert
$values = rtrim($values, ",");
$sql .= $values;
$stmt = $conn->prepare($sql);
$stmt->execute();

This decreased my average insert to less than 10ms per line inserted (as opposed to 300ms to 1sec per line previously). Never thought the difference would be that big. Thanks.