Hello,
Optimization Magento site will take a lot of time. You should calm down and google. There are some good points you should care:
-First of all, your site enables Magento Cache?
-Second, You can use Google Pagespeed to test your site: https://developers.google.com/speed/pagespeed/insights/
Google also has some guides to optimize your site. Google Pagespeed focus on Image, Css and Js. For example, for image: you can see here: https://developers.google.com/speed/docs/insights/OptimizeImages.
-Third, another reasons is Coding. Magento is a complicated platform. If your developers aren't good enough to custom Magento site, they can write "bad code lines". You can read more here: http://magento.com/blog/best-practices/top-5-magento-coding-recommendations-optimize-site-performanc...
Last but not least, you can read more here: http://www.gxjansen.com/101-ways-to-speed-up-your-magento-e-commerce-website/
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