@taner 

Make sure you are on the lastest driver.  Check your connection rate, this will determine the eventual speed.  Move closer to the router (if possible). reduce interference such as if you have this in a PC in an enclosure make sure the back is open or place the PC ontop of the desk if possible.

Distance and interference testing starts with testing speed next to ther router and then moving a few feet back and retesting and then repeating till you see signficant drops in speed and performance.  This is where interference, congestion and network enviroments create the low performance you are seeing.  Some of this can be changed by postion and location, some by changing to a less congested channel, but other are just simply due to the network.  

@taner Did you find any solution ??

I am having the exact same problem and my router is fixed in wall to next room hence there is no option to relocate my desktop or router,

so currently i am just thinking of returning the product if i cant troubleshoot it.

please reply soon if you have got a solution