Hello Giuseppe, its pretty normal, cause maria engine have a log in the Data file who make insert, update and replace statements slower than Myisam, its secure th data enabling restore to any point in time to your database table.
check the filsize in the data directory and you will understand better
the FRM files both myisam and maria have the same size but
the maria data file is bigger than the myisam one and also
the myisam index file is bigger than the maria index file.
if you need increase this relationchip tri to use myisam to insert fast and maria do insert reliable and fully recoverable (including deleted rows) table.
i supose it can help you.
best regards.
Michel Souza
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Atenciosamente
Michel Souza