Short answer is since there are multiple ISP in any one city there is no easy way to predict the performance between 2 users. In the worst case 2 neighbors using different ISP may have to go all the way across the country to connect.
To even start to explain this you need a good understanding of how internet routing actually works.
Go look up the topic of peer points. You can then use some of the tools ISP provide and with some good knowledge of BGP routing you can figure out a lot of how things are connected.