[RT-N66U] Slow internet and streaming on Wi-Fi but fast with ethernet

Hello, I bought the Asus RT-N66U since I read many reviews of how good it is. I've been using it with pretty much factory settings and the speed is very good when connected with ethernet. However, Wi-Fi speeds are very slow despite getting a strong signal; video streaming such as YouTube would constantly buffer and lag for a laptop that's in the same room as the router!

My internet plan is with Time Warner and my plan gives me up to 100 mbps download speeds which I get through devices...

[RT-N66U] Slow internet and streaming on Wi-Fi but fast with ethernet