Depends which VPN you are using.Problem with vpns well what I've found anyway is all the servers are overloaded and a lot of user don't have enough speed on their home Internet to take a hit from a vpn.
Not all cause a drop in speec but most do.
Testing on many people, only lost about 5/10 mbit of speed when on vpns.
Wide range of suppliers of vpns, But yes depends on supplier.