Can a Range Extender slow my internet speed ??

I have a TP-LINK, TL-WA850RE range extender set up between my router and computer. In other words, my range extender picks up my internet signal wirelessly, then the signal is sent to my computer through an Ethernet cable. I have Mediacom cable internet service, and am supposed to be getting 15Mbps download speeds, and am only getting 6Mbps. There's only a 15 foot distance between the router and range extender, and no walls or any other obstructions to interfere with the signal. I just got there service installed a week ago, and am wondering if the range extender could be the problem, or if I need to have Mediacom come back to fix this problem ??