Hello,
By optimising JS and CSS in T3 you are only helping with the serving of the template files.
There are many things to consider to help speed up your site, such as the number of modules being displayed, size of images being rendered, the about of database queries, these are just some examples.
There are many online tools to run speed tests and analyse page loads that will help give advise.
Do you cache on, or GZIP, you may find them helping too.
Also consider looking at the main Joomla forums, unless you do suspect the template is slowing load times, personally it loads pretty well.