there is only so much spectrum, for any given frequency range you can transmit only so much data in any given volume. The further between transmit/receive the weaker the signal inverse-square.
So in a high density population space (E.g. apartment block) your physical distances are short so you'd have less of the "benefit" of the inverse-square reduction congestion, so competing with the congestion the signals from nearby overlap. So a battle.
The way to improve that battle is for everyone who has 5Ghz capable routers / devices to move to a higher frequency, like 5Ghz. Then the higher frequency is traveling less distance, not so much through walls you'd have in an apartment block and so your local 5Ghz is competing with a weaker neighbor 5Ghz then the same situation 2.5Ghz. So you'd get more to you bandwidth resulting.
In my house those devices which do good 5Ghz I don't even enter the 2.5Ghz router bandwidth so these 5Ghz devices release up the most 2.5Ghz spectrum for everything else. My Chromebook has amazingly good 5Ghz and its what I do most of my Wifi streaming on.