Wi-Fi Routers

I have been using a tp-link brand router for over 2 years now. I have recently been experiencing a lot of lag times. When I checked my download speed, it was 104 when wired directly to the modem. Through the router,d it was only 42. I contacted tp-link and was told by them that their routers typically deliver only 40-50% of mbps. I find this very hard to believe. Any advise?