Magento ® loads a lot of javascript libraries which increas load speed. Usually it increases by little but if you have custom template it may include additional files and take more time . But when you are attemting to minify or combine magento default javascript files it gives errors on the checkout page. If you still have issue with it please review compiled file which working for magento 1.6.x-1.9.x whith defualt files inclued at the article page.