I got a pcie wifi card and after i mounted the exstension cable for the wifi antenna and moved it to the desk facing the antenna against the router then i got my high download speed again. But its sound like thats not the case for you, It sound like a software issue. Try disable anthing that u think might interupt, like firewall etc
Have you tried more then one "download speed test" ? Cause i notice a diffrens between them, just to ensure thats you infact can get the speed your paying for.