Whoever said WiFi is only as fast as the slowest device doesn't know what they're talking about. If this was the case, then Wi-Fi is a terrible choice of standard for networks.
The first thought would be checking how reliable your signal is and what's between the end device and the router. If you have an Android device, Wi-Fi Analyzer
is a great app for determining where the gaps are in your home.
As far as things between the router and end device, any large metallic object is a reflector and will hinder the signal. I stayed at a place once where the router was no more than 20 feet away from me, but I had a really crappy signal because between me and the router was a large bathroom mirror.
Also I've found the Windows tends to lie about signal strength and actual network reliability. But this may be a problem inherent in wireless networks in general.