Re: query execution becomes slow after the first execution of the program

[Rick James]

Please provide
* SHOW CREATE TABLE tbl\G -- engine, indexes
* SHOW TABLE STATUS LIKE 'tbl'\G -- sizes
* SHOW VARIABLES LIKE '%buffer%'; -- cache size <-- I suspect the answer is here
and surround them with [ code ] and [ / code ]

Also, consider "batching" the inserts. It is messy in PHP (can't use <?>s), but much faster to execute:
INSERT INTO xx (a,b,c) VALUES (1,2,3), (4,5,6), ...