WiFi speed and AirPorts

I currently have an AirBook and two relatively new, AirPort products, an AP Extreme and a Time Capsule, all of which support 802.11ac, and my network works fine and fast. I'm thinking of adding an AP Express (for AirPlay on the TV). The newest Express only supports 802.11n, which is slower than 801.11ac. I recall reading somewhere that the speed of a network is limited by the speed of the slowest device on it, which would mean that the Express would slow me down considerably (about half, I think). Is this true? Or can I use the Express, accepting that AirPlay on the TV may not be as fast as could be, but everything else will still run as fast as before?