Does a VPN slow down your internet?
A little, inevitably: your traffic takes a detour through an extra server and gets encrypted on the way. With a good VPN running a modern protocol like WireGuard you will barely notice; with slow or overloaded services the difference can be substantial.
Every VPN costs some speed, because your traffic travels a longer route and has to be encrypted and decrypted along the way. The question is not whether you lose something, but how much. Modern protocols like WireGuard (and variants such as NordVPN's NordLynx) are efficient enough that on a typical home connection the loss is often a small fraction of your bandwidth.
Three factors decide the difference. Distance to the server: one nearby beats one on another continent. Server load: free services are often congested. And the protocol: in the app, always pick WireGuard or the provider's modern equivalent over legacy options.
For everyday browsing, email, and video streaming, the difference with a good provider is barely noticeable. Only on very fast fiber connections or in competitive gaming does it become visible.