That's why professionals don't call this "wifi contamination", we call it spectrum utilization/congestion. The problem isn't the number of wifi APs, the impact of beacons (i.e. idle APs) is, while not zero, quite limited and only visible in extreme cases. The actual problem is traffic, which consumes available spectrum when being carried.
It's a factor of RF bandwidth, time and space. With some non-obvious parts:
- setting your TX power too high makes you consume spectrum in a larger area, harming your neighbors. Don't yank the TX power to maximum just because it "feels" like that should be better; there is no difference between MCS (= speed/rate) 11 with 10 dBm headroom and MCS 11 with no headroom, you get ≈120Mbit either way.
- conversely, using old APs, devices, or stretching the wifi connection too far consumes excessive spectrum since you'll get a bad data rate and use much more time to convey the same data. Due to this, a repeater can in fact improve performance for devices not even using it, by getting rid of low-MCS traffic.
- don't use wide channels when you don't need the performance. A 160MHz channel means 160MHz width of picking up interference. While chipsets are somewhat intelligent about this, if you're fine with ≈200 Mbit (single MIMO stream) there's no point in going wider than 40 MHz.
- multicast is death. It's a very common wrong belief that wifi requires you to send multicast traffic at the lowest possible rate. It doesn't, but almost all low-end implementations are lazy and do just that. "Lowest possible" in this case means the lowest rate the BSS configured to support. If you have 802.11b enabled, that's generally 1 Mbit/s. Disable that, and you get the lowest 802.11n rate, which is ≈6.5 Mbit/s. If you need to deal with a lot of multicast, disabling some low MCS might also be worth it to raise that even further, but then those MCSes are not available to cover far-away devices anymore. But then again you may not want that to begin with (see above).
- it's highly dependent on building characteristics; thick stone/steel walls block much more RF energy than drywall or wood.
- if you can, just use cables. If it doesn't help you, it might still help your neighbors.
So… yeah, a lot of people consume media in the evening, and that does make it much worse.
P.S.: MCS indices: https://mcsindex.com/