This typically depends on several factors, first and foremost, the quality of the VPN service. The best VPNs will have servers with unlimited download speeds and will avoid throttling (the intentional slowing or speeding of an internet service by an Internet service provider). The next factor to consider is the distance between your physical location and the server to which you are connecting. If you pick a server that is relatively close to you, your internet speed will be faster than if you had picked a server halfway across the world. Additional factors include the level of encryption and protocols you select.