How can I improve the performance of this PHP / MySQL code?

The following is part of a larger PHP script that adds or disconnects users from a MySQL database using a CSV file and a client ID as input.

There are two tables, users and users_clients . Later, communication is maintained between users and clients, since a user can belong to several clients.

This is a table structure

users (she has more fields)

id        | int(11) (primary key)
user      | varchar(100)
pass      | varchar(100)
category  | int(11)
date      | timestamp
name      | varchar(100)
email     | varchar(255)

users indexes

SEARCH    | user        | FULLTEXT
SEARCH    | name        | FULLTEXT
SEARCH    | email       | FULLTEXT

users_clients structure

id_user   | int(11)
id_client | int(11)
status    | enum('active','inactive')

This is the main script flow for adding each user from a CSV file:

• , .

  SELECT 
      LOWER(user)
  FROM
      users u
      INNER JOIN users_clients uc ON u.id = uc.id_user
  WHERE
      u.user = '$user'
      and uc.id_client = $id_client
  

• , .

  INSERT INTO 
      users ($fields,id_client)
  VALUES 
      ($values,'$id_operation')
  

• . , - mysql_insert_id, ?

  SELECT
      u.id as id
  FROM
      users u
  WHERE
      u.user = '$user'
      and u.id_client = '$id_operation'
  

• .

  INSERT INTO
      users_clients (id_user, id_client) 
  VALUES
      ('$id_user','$id_client')
  

400.000 . script 10 + 5 CVS 500 .

, ?

.

PD: , pastebin.