Great Question. Caching can effect your rendering time. If ''Simulating a new user'' in the runtime settings is enabled then this will greatly effect your timings. We recommend unchecking this setting and running your script several times. You should see better rendering times that match manual rendering times. In Addition, HP has stated the following in regards to AJAX TruClient and Web rendering times.

HP has provided the following:
This is because of the following reasons:

1. AJAX Truclient is GUI Based recording and supports mouse clicks etc, whereas Web HTTP/HTML is HTML based recording or URL Mode.
AJAX Truclient uses GDI Resources whereas Web does not use it. Reference on GDI: http://en.wikipedia.org/wiki/Graphics_Device_Interface

2. AJAX Truclient does client side processing and page rendering of objects such as DOM with JavaScript, whereas Web HTTP/HTML does not do any client side processing.
3. AJAX Truclient has to do JavaScript evaluation on the client for DOM objects that takes time for rendering a page.
4. AJAX Truclient needs to have ''replay think time as recorded'' in runtime settings, otherwise you may see step download timeout errors since this value is 120 seconds by default and it is not enough to get the page and render the page on the client.

5. AJAX controls are enabled in AJAX Truclient but they are not enabled in Web HTTP/HTML mode.

I have the same issue for AJAX trueclient time Vs firebug..these two numbers never match
For example if we take a product page in some shopping websites and compare the results with firebug and trueclient
Firebug gives 3.4 sec and Ajax trueclient protocol gives 0.9 seconds....with end event step network completed.
This is difficult to say truclient provides accurate numbers...any help will be appreciated.
I feel trueclient is only calculate onload time but not rendering time..