Optimization is a very big and complex task in IT system. Almost every tier or level can introduce the delay observed. Based on my working experience I can provide below but it is not complete by any means. Other veteran IT developers may want to add on.
1. Assume your CMS do talk to a database, check the SQL statement. Which SQL statement take the longest to finish. Check which SQL statement although execute very fast is called very often by the application layer.
2. Application layer sometimes depend on the programming language. For Java and PHP or others, garbage-collection can cause a noticeable delay if the application allocate memory in great abundance without resort to pooling/caching.
3. Application layer design also matter. If a task need to go through multiple layers due to OO design, it can matters especially if that task is called very often throughout the whole application life-time.
5. With AJAX catching on fire, developers may get carried away by making extensive use of this technique to provide a more "smoother" web browsing experience but at the back-end HTTP request is "bombarding" the server and the server will be very busy serving all those request.
So essentially I group them into 3 layers. Client, Application, Database. Each layer need some investigation on it alone.
The thin-est client layer I could think of will be those Mainframe dumb terminals with the green characters on the black monitor. I cannot even use mouse to click on the screen
The first step towards finding the cause would be to figure out if the slowing down is in the browser or the server. From what you've said, I'd be inclined to think it's the browser (especially since the browser is IE )
Does this problem only occur when a user is logged in to the CMS as a user, or does this problem happen even if the user is just loading through page after page without being logged in? Is the problem always fixed by closing the browser, or does this slowing down sometimes persist even after the browser has been restarted?
If the problem is always fixed by restarting the browser and it happens regardless of whether or not the user is logged in, then it's almost certainly a problem with the browser.