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Whenever possible, use an opcode cache (or ask your web host to install one for you), in order to alleviate the server's processing load. Opcode means "operation code", and defines the compiled state of the dynamic files, which can then be processed faster.
PrestaShop is compatible with eAccelerator (http://eaccelerator.net/) as well as the new OPcache feature from PHP 5.5.0: http://www.php.net/manual/en/intro.opcache.php.
Improving MySQL performance
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