It looks like everything related to performance has multiple dimensions. Reading recently excellent posts A non-geeky guide to understanding performance measurement terms by Joshua Bixby and Building a High Performance Website by Phil Stanhope, I realized how many dimensions even a relatively simple term “response time” has. And, moreover, it looks like we don’t have a reliable way to measure the response time that would matter to end user (I guess something between “time to display” and “time to interactivity” depending on the site design, if follow the posts terminology). Both authors look at this rather from the front end / Web Performance Optimization (WPO) point of view.
Spending most of my time in performance testing, I’d guess that “response time” comes from load testing / active monitoring tools that are the main source of performance information (the “waterfall” approach of the WPO community quickly becomes popular – but I am not sure how many monitoring services use it). And in this case, “response time” is what the tool reports. What “response time” means in such case is heavily depends on the tool and its settings – and in many cases, I guess, it won’t be any of the metrics provided in the aforementioned posts (which, I guess, are standard in the WPO community – but they may be not easy to measure by load testing and enterprise monitoring tools). For protocol-based tools it would be probably the time of receiving all requests without any client-side activities (with many additional details of browser emulation- like caching, threading, keep-alive, compressing, etc.). For GUI-based tools it probably depends on what underlying mechanism the tool uses and how the script is designed. Quite often if you don’t set any specific checks it may report a success without full downloading and rendering (and when somebody say that a modern sophisticated site will load for 0.169 sec over the Internet it would be my first guess). Although, if scripted properly, it perhaps may measure the performance metric that matters (when the page would “be almost fully interactive”) by checking that the parts that matter are downloaded and rendered (that probably can’t be done without manual scripting / analysis).
That brings an interesting question about Application Performance Management (APM): what End-User Experience Monitoring (EUM) a.k.a. Real-User Monitoring (RUM) measures? EUM/RUM is considered as an integral part of APM (and definitely should be), but may measure pretty different things depending on the approach to measure it. And as I mentioned above, it probably won’t be the actual end-user experience – but only its approximation by another metric (different for different tools).
Only thing that often saves us from all this complexness is , as often happens in performance, that in many cases it doesn’t matter. All of the metrics are just close enough from the practical point of view. In old good times of plain html the main part was getting response from the server, the client-side part was fixed and usually small. So it wasn’t said much about different kinds of response times in the past. The situation is changing now: the front-end time becomes significant (see the Performance Golden Rule by Steve Souders, keeping in mind that it is based on front pages mainly) and now it looks like we can’t ignore the differences between response times anymore.