Thank you, I found the issue !!!
https://expertincrm.medium.com/how-to-improve-the-speed-of-sugarcrm-instance-657449fd1ad7
“First of all, make sure that all of your tables are the same character set. If they are not, this can greatly slow down queries that join between tables of different character sets.”
Turns out my most recent custom tables had the character set “utf8mb4_general_ci”, while older tables use “utf8mb3_general_ci”.
Changing all characters set to “utf8mb3_general_ci” solved the issue, Temps de réponse du serveur : 0.06 secondes 
command used if anyone needs them :
SELECT TABLE_NAME, TABLE_COLLATION FROM information_schema.TABLES;
ALTER TABLE table_name CONVERT TO CHARACTER SET "utf8mb3";