Problem is not in whateverG. Problem is in uplinks from base stations. Large cities does not have enough bandwidth between stations and internet to properly serve a lot of clients. Small cities have less cliens, so uplinks are not so overloaded.

Cellular network is not just your phone and some abstract Google or Apple services. It is a multilayer system with many bottlenecks and places where something could go wrong. 5G/4G/xG is only tiny part of it and not the one that exclusively define overall performance.

Changing oldG to newG will change absolutely nothing for user without respective rebuilding of other layers nobody bother about since 3G introduction.