Mobile phone internet USB tethering very SLOW
Mobile network carrier often have transparent gateways installed all mobile network traffic have to pass. Such gateways are able to "detect" (smart guessing) if traffic comes from the Smartphone or a tethered PC. 

Depending on the data plan contract, tethering may be prohibited and in such a case the carrier can just forcefully slow down the tethered connection.

As only the tethered connections are slowed down you can end up in a situation where identical downloads get a totally different download speed.