iPhone drastically slowing down wifi
You'll want to check the diagnostics offered by the router.

In some cases your router could be "beam forming" to maximise bandwidth to a single device (such as a laptop). If you have relatively bad conditions, this could be necessary to get good speeds. By bad conditions I mean for example neighbouring WiFis networks interfering, noise from other equipment, big distances between router and device or objects blocking the signal (walls for example).

If you add another device to the network, such as the iPhone, the router could be forced to stop "beam forming" because you have too many units connected or similar. That could give you worse performance on the existing units, even though the iPhone is not transmitting/receiving any actual data.