My uncle told me that if a slower device connects to a wireless network, the entire network slows down more than can be attributed to the bandwidth being split even further. So a network with 2 existing connections would have the existing connections slowed down more by an iPhone 4 connecting than an iPhone 6 connecting because the iPhone 4 is slower than the iPhone 6.
Is this true?
I do not know if this applies to n networks, however for networks that support b and g I do know that connecting a older device on to a network that does not support g will make the access point run at b speeds only so all your devices that do support g will only communicate with the wireless access point using the slower b protocol.
The iPhone 4 and 6 both support g so this problem would not happen with those two devices if they where the only two devices on the network.
Connecting a slow device to the WIFI network will make it slower, but it will not slow down the entire network to the speed of the slowest device.
In simplified terms when the slow device is transmitting or receiving it means other devices can't use the space allocated to the slow device, which takes longer to do anything because its slow. Similarly there is additional overhead on the network to support the older slower protocol.
