Store Procedure - SLOW insert + possible VARs conflict

Question

I'm working on a messaging system that basically consists of two tables: CONVERSATIONS and MESSAGES. The procedure checks if there is a conversation, and if there is it saves the conversation ID into a VAR and uses that for the message insert. IF THERE IS NOT it creates one entry in the conversation table and uses the LAST ID as an entry value for the MESSAGES table.

SO far it works just fine but I have 2 issues:

• In my ignorant view the so defined VARS @last_id and @conv_id may incur in some sort of conflict IF more users are calling the procedure at the SAME TIME. Imagine @last_id is set to 40 and before the insert happens an other user calls the same procedure and that value gets set to 41.. Is that a possibility???

• This INSERT procedure seems to be PRETTY slow, once cached though it seems to get a bit faster but I'm not happy with it. Is there any solution beside INDEXING?

Thank you.

DELIMITER //

CREATE PROCEDURE chat_insert(id1 INT, id2 INT, text VARCHAR(250))

BEGIN
SELECT id INTO @conv_id FROM conversations WHERE 

(user_id_one = id1 AND user_id_two = id2) 

 OR  (user_id_one = id2 AND user_id_two = id1) LIMIT 1;  

IF(@conv_id IS NOT NULL)    

THEN

    INSERT INTO messages (conversations_id,user_id,text) 
     VALUES (@conv_id,id1,text);

    ELSE

            INSERT INTO conversations (user_id_one,user_id_2) 
            VALUES (id1,id2);
            SET @last_id = LAST_INSERT_ID();
             INSERT INTO messages (conversations_id,user_id,text) 
            VALUES (@last_id,id1,text);
    END IF;
END 
//