[cflsystems]

If you want to do it right you have to re-write all XC templates. Take out the hundreds of nested tables, convert to html5/css3, optimize css (you can have 50%+ less css, the default one with XC is very bloated), optimize js (you don't need a lot of it if site uses html5/css3), make sure though you keep some of the css classes/ids as a lot of modules and XC jquery uses is...
It is a lot of work but at the end you will have nicer, cleaner and faster code