Very light queries are in slow-query-log ! Why?
I noticed you are searching for an IP address

You could add an additional column with a numeric value for the IP

First, create the column to hold the numeric value of the IP and index it

ALTER TABLE gl_visiteurs ADD COLUMN IPVALUE INT UNSIGNED DEFAULT 0;


Next populate that new column and index it

UPDATE gl_visiteurs SET IPVALUE = INET_ATON(IP);
ALTER TABLE gl_visiteurs ADD INDEX (IPVALUE);


From there, you can do deletes via numeric lookups like this

delete from gl_visiteurs 
where IPVALUE = INET_ATON('68.180.230.33');


and do inserts like this

INSERT INTO gl_visiteurs (IP, IPVALUE, user_id) 
VALUES ('68.180.230.33',INET_ATON('68.180.230.33'),0);


UPDATE 2017-01-09 17:57 EST

Based on your comment, you can drop the IP column and keep the IPVALUE column

Then your inserts would be

INSERT INTO gl_visiteurs (IPVALUE, user_id) 
VALUES (INET_ATON('68.180.230.33'),0);


but to see the IP your selects must be something like

SELECT INET_NTOA(IPVALUE) as IP,... FROM gl_visiteurs