Why LR tool is not able to capture browser rendering time?
How can I capture the browser rendering time?
Loadrunner is a tool that captures at a level below the browser (for example at the http level). It isn't a browser, and thus doesn't do any rendering that could be captured.
There are tools that hook into the browser itself (such as HTTPWatch). see: http://blog.httpwatch.com/2008/05/14/using-httpwatch-to-measure-page-load-times-for-new-and-existing-users/