Wifi devices with a poor signal or too much interference automatically switch to a lower modulation rate. Lower modulation rates are slower but more reliable. Look at this screenshot from an old Ubiquiti Nanostation Loco M2 radio for example:
Those are its supported modulation rates at a 20 MHz channel width which is what most normal wifi devices use. At the highest rate, it can achieve around 130 Mbps throughput. But this only works in ideal radio conditions. Interference or poor signal causes transmission errors, and the radios respond by reducing the modulation rate. In some cases a device can even interfere with itself, like if some metal object reflects a signal so the receiving station hears an echo. This one reason why sometimes you see a great signal and the device says it is connected at 130 Mbps or whatever, but the moment you start really using the network, the speed can drop like a rock. You can read more about modulation rates here:
IEEE 802.11n-2009 - Wikipedia
So then you start using wifi cameras, which generate a steady stream of data typically below 10 Mbps. In ideal conditions, this would take less than 10% of your network's airtime. But wifi cameras are rarely in ideal conditions. Often they are mounted outdoors so they have an exterior wall to punch through, not to mention any interior walls. Often, there is more than one camera and they might be mounted on opposite sites of the house. Wifi devices normally listen before transmitting in order to avoid interfering with each other. But if your cameras can't hear each other, they are far more likely to interfere with each other by transmitting at the same time. Anyway, your network ends up spending most of its time at a very low modulation rate, and it is therefore taking most of the available airtime just to get a couple of relatively small streams through.
