My mom just bought an Airport Extreme router recently, and she said the salesperson at the Apple Store told her that all devices on a network run at the same connection speed, and that speed is determined by the slowest device on the network. So if you have a shiny new Mac that has the potentential to run at 1 Gbit/second, you'll be slowed down to a 10 Mbit/second speed, just because you have your old phone connected to the wifi, downloading some apps at the same time. This doesn't really make any sense to me.
(Please don't speculate, I can do that myself. Answer if you have experience with this topic).