Page cannot load fully until all JS and Head load. once all scripts loaded then the content of page shows in top to bottom order of page contents. so it is necessary to reduce the scripts into head tag. use small amount of css and js script so that page will load fast. Also its good practive to include java scripts at the bottom of page or at footer so that its load last and user see the page. large size of css or javascript take some time to load.]

In Drupal 6 there are many js files in core in modules, misc directories. Also many js files are load into HEAD which are not even used. developer should configure such js and css files so that page loading speed will make fast. in case of css same method we have to apply as above.

Optimization of js and css required once we finish development. remove non-essential characters from code to reduce the size of script so its load faster. remove comments, link breaks, tabs. good practice to use some compressor tools to compress the javascript and css file once you finish the development.

In Drupal On the "Performance" page ("Performance" - admin admin/settings/performance) there is an option to enable/disable the optimization of CSS and JavaScript files.