Will adding a Wifi repeater/extender slow down the wifi speeds?

I have AT&T U-Verse high speed internet, the 2WIRE modem/wireless router they provided is good, but does not cover the whole house. So I bought a repeater. The connection is supposed to be 16mbps, but now that I have added the repeater, my devices set up on WiFi are significantly slower, is there a way to fix this?