Hello!

First of all, website response time depends on many factors(network configuration, what data are you accessing, does website calculates data or just returns it, etc) and the only thing you can do is calculate average website response on a particular request(because it may differ for different URL's). So, tracking the time when the request was sent and the time the response was returned, you may have a set of data, on which you can count an average time.

If we are talking in terms of TTL property of a package(lets downgrade to network layer of OSI network model), you can consider the response time as a number that was extracted from TTL property(for both request and response). 

To see package properties, you can use many different tools, like Fiddler

Regards,

Artemiy Firsov

Regarding Apache JMeter specifically, as far as I know, it measures two parameters when determining the load time.

First, it attempts to measure latency. "JMeter measures the latency from just before sending the request to just after the first response has been received". This is what the official documentation states.

The second is the actual time to process or the complete load time. This is the time needed to process the request from when it has been made. This time includes the Connect time (in case of SSL the hand-shaking done as per the protocol requires it. JMeter completes numerous retries of the same "sampling" and calculates the Standard Deviation. But, it calculates the population standard deviation, and not the sample standard deviation as one would assume.

At the client side, the in-browser component of the process measures the rendering time. This measure depends on the performance of the hardware and software configuration of the platform that executes the in-browser rendering of page components, thus it is weighed much lower than other parameters of the final measurement. Nevertheless, it is still included in the final measurement if the user outs it in with settings switch arguments of the JMeter configuration.

You can find more about this in the official Apache JMeter documentation.