Encrypting and decrypting your traffic, as well as routing it through an extra server, is a lot of extra work, and if you don't pick a provider with decent speeds, they'll likely drop your performance.
Top VPN providers keep this to a minimum by using well-designed apps and the most efficient protocols (the rules a VPN app and server use to transfer data). We regularly see the best VPNs reach over 600 Mbps and more, so if you choose your provider carefully, you may not even notice any difference in speed.