When trying to optimize the performance of the theme one needs to think about both about the time spent on the server as well as the time on the client side. On the server side it is all about execution of code on the server that can take up processing power and memory. This can be analyzed with profiling tools like the Java Health Center or Jprof or … Also the number of requests influences the server side response - if an excessive amount of requests is sent it will slow down the server.
Tools like Firebug or Development Console from other browsers typically allow to determine how much time is spent on the download as well as the rendering of the resources.
My recommendation is to check the time spent for the loading of the page and then optimize the slowest parts. If the main Portal request (typically to .../wps/myportal/... unless you changed the context root) is taking too long you would need to check what is slow on the server side. Most of the other resources being rendering are referenced by the theme or the portlets on the page.
This article does not cover basic tuning tips like ensuring that compression is enabled on the web or proxy server and that the correct cache headers are being set for static resources to prevent subsequent downloads of static resources. Both tunings have a large impact on the theme performance and should be enabled. Please see the Portal tuning guide for more information on those topics:
IBM WebSphere Portal V 7.0 Performance Tuning Guide