EDIT I run the pi (either a pi 2 with external wifi dongle or pi 3) as servers and WAP for a local server project (no internet connexion involved here). I have noticed that the performance of the pi as a WAP if the limiting factor of my system.
I've been testing the wifi performance by connecting up to 15 tablets and measuring lags between the time the request is sent by the device and the time it's received by the server, and other way around. But it sounds like a very inefficient testing method.
Is there any alternative to connecting more and more devices until it starts falling?