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site is hosted on a linux vps (php/mysql)
and is the latest joomla version. 2.5.7

can anybody give me tips?

Do you have Joomla’s page caching enabled? Odds are it is a performance issue with the database (is it on the same server as the web server)?

Since it’s a VPS you should be able to install New Relic or something similar which will let you profile your pages in production and point out the issues (slow database queries, external calls, etc).