WMF metrics meeting". One of the interesting talks starts at 37:46, it is Ori Livneh talking about the site performance of WMF projects.
The bottom line is that page load time matters and, that it can be improved. The type of optimisation Ori is talking about is latency where distance from the WMF servers is not taken into consideration.
A lot of work has already gone into optimisation; one of the reasons for introducing LUA is that it performs so much better.