There are several questions which come up when you are performance testing your application using IBM Rational Performance Tester (RPT). Among all such questions, let's discussion on question like "How to measure the client side page rendering with RPT?" You might say, that you script measuring only the downloading of multiple request/ response for the specific click event and its not capturing the service level agreement statistics to display the downloaded request\response as per the browser level. It could also be a case where, when a script is recorded in RPT, the manual navigation from one page to another takes a lot of time. However, when the script is played back, the response times vary less when compared to what you see manually.
Now, lets look at this from RPT perspective.
IBM Rational Performance Tester is a server load testing tool, designed to measure effects of multiple users on a server. As such it does not make use of a client side browser instead simply interacting with the server
through "raw" HTTP protocol and the data you're seeing in reports is geared toward the server perspective (things like page response time). The time it takes the server to get the page to the client is the important piece of a server load test. The time it takes the client to render that page isn't relevant in a strictly load testing sense.
As you know, RPT is used to measure the response time between the fist byte sent in the page's first request and last byte received during the response from the server. The client side experience (how long for a page to be
loaded) may not reflect to the response time data. The time may be in a part of think time which will be excluded in response time calculation. For example, RPT does not include page rendering in the response time that is displayed in the performance report. However, it should be noted that there are potentially "delays" in the script that could increase page response times. Think times are associated with pages and are intended to represent human pauses during recording - these don"t impact page response times. A reference article can be visited here.
So in short, there exists no straight forward way to get the measurement metrics pertaining to the client side browser rendering in RPT. However, The client side rendering may be captured as 'Client processing delay' in the recorded test. There may be counter(s) that measure the net server response time but there could be no counters for client rendering. That is understandable because there is no "client" and so no "rendering" that actually takes place when a load is being driven by RPT.
Here's a short note on the impact of client processing delays which might be of further help in understanding.