I know that, but my friend have other ISP with 20mbps too, and with ethernet cable he gets 6mbps, so I think that I can be because my ISP, but maybe you are right...Downloads are in megabytes MB and internet speed is in megabits Mb -- Your 20 Mb == 2.5MB You are getting your full speed.
It says 20mbps with ethernet and same with wireless, but Im getting 2,4mbps...Have you run a speed test to your ISP's test web site or another test web site with a server in your local area? This sounds like your ISP is not giving you all of your bandwidth you are paying for.