I don't have Starlink but this is how I believe VPN's generally work. Turn it on and your internet is then routed through a server that is associated with the VPN provider. The data is then encrypted and forwarded on to you through to you device running the VPN software. This is were it gets decrypted and processed by your device. When sending data out the same thing happens but in reverse. I don't see why it would be a problem with Starlink, it's not doing the encryption and Starlink is said to be compatible with VPN's. The problem is with the computing capabilities of your devices.the slower your devices are the slower the decryption can be. For general things it could be fine and you wouldn't notice any difference. An example using my internet is i have ultrafast 1000mbps speeds, usually around 900mbps actually, with a speed test VPN off. With the VPN on around 350mbps on one device or 270mbps on another device, both connected to lan ports. The difference is the processing power of the device. Using my phone with 4g connection VPN off 138mbps, VPN on 110mbps, slight difference there.
I have used both PIA and NORDVPN and have found them both to be good but NORD would be my first choice.