Q:

How to debug a website template which is very slow (client side)? 

A client has purchased a website template and wants me to work with it.  However, for whatever reason, the template is very slow and makes the browser hang / slow down, particularly when scrolling down. 
The issue is in the client side code (javascript, flash, images, or CSS), and not on the server side. 
How can I debug this site to try to determine exactly what is causing the template to be slow? 
I've tried adding / removing content but it doesn't seem to help, so I think that it might be linked to the css

A:

Use a modern browser, press F-12, clear the cache, open the NETWORK panel and reload the page.  This will give you a list of resources, how big the files are and how long they take to transfer and load. 
The usual suspect: images over 100K that comprise UI elements.  I had a friend make a banner that was 2500px wide, but resized it in the CSS to be 960-wide.  It still used the same 350K PNG file.