Hi,
First off, I am not sure that it is possible to measure the time between two arbitrary requests in LoadComplete.
But I am also not sure that your idea will provide you with the result that you really need.
If I understood you right: assume that the time between two requests in question is 0.5 sec. But in order to become responsive, the page needs to download several more megabytes of data and execute a dosen of jscript functions which takes another 5 sec. Even if I, as an end user, can see somehow that the transaction was executed during 0.5 sec, does this help me somehow considering that I still will not be able to perform any action on the page for the next 5 sec?
For me, the more correct approach is to use the server-side counter (see help for more details) to measure the transaction time on the server and to use LoadComplete to measure the overall time between the moments the transactional request was sent and the page became responsive again.
If you need, you may also profile the web page using AQtime profiler (https://support.smartbear.com/viewarticle/43013/).