Hello,
You can check if you're using APC in the PHP config info or using the phpinfo function (http://php.net/manual/es/function.phpinfo.php).
About MultiMerge class, please check the API doc page:
http://www.phpdocx.com/api-documentation/docxutilities/merge-Word-documents-with-PHP
About your server, we mean that maybe you could run it on a machine with more RAM or better CPU, as we don't know you server specs.
Regards.