I'm sure
@Edwin will know more about this and hopefully he'll share; but another factor I think is crucial is contention. For example, I'm in an apartment complex in China right now and in the morning, the speed is reasonable. As soon as the day wears on, the kids get online and start playing their multiplayer games, and the parents are all online watching videos. Then the speed starts to trickle. Same in Thailand.
When I lived in the UK I had a BT connection. When I asked them about contention they told me there would be a maximum of 50 'sharers'. I don't have my Android phone with me and it's not available on iOS, but a really fascinating app is 'Wifi Analyyzer' which shows you all the wifi routers in range and their signal strength. There are apps for iOS but they only address public systems like Starbucks etc., not private 'in-range' systems. If you have an Android phone - grab this app - very interesting. You can also use it to choose the optimal channel spectrum to set on your router.