Most likely you will see a moderate speed decrease to your internet while using a VPN. This is mainly due to the process of your internet traffic being rerouted and encrypted through a VPN server before heading on to its destination. The amount of latency you will experience through this process is down to what country's server you choose to proxy your traffic through. If you choose a server located on the other side of the world, then you will notice a much greater hit than if you were to connect to a server nearby in your same country. Thankfully, most modern VPNs should have the infrastructure to maintain high-speed, secure data transfer rates. So it's likely you can still browse and stream with no noticeable impact while using a reputable VPN like those listed above.