I have a similar connectivity issue on my Samsung ATIV Book 9 Plus running Windows 8.1.
My Wi-Fi connection slows down considerably to under 3Mbps from around 50 Mbps that I have.
As many other folks already mentioned, I do have several other devices in my network where this problem doesn't occur (IPad, Win7 system etc.).
After rebooting the my laptop, everything goes back to the normal (50 Mbps).
Thing is that running release/renew IP won't help.
Definitely this is a Windows 8.1/8.0 issue/bug that needs to be fixed by the Microsoft folks.