Gluetun is a container to allow vpn connections. In docker any container you want to use the vpn connection you would put network_mode: service:gluetun and those containers would then use the vpn connection. With tailscale you would still access those containers through your tailscale ip address but they will be behind the gluetun vpn container. Tailscale doesnt act as a vpn as in masking your online activity it only secures your server from being accessed from outside of your tailnet. You can enable mullvad in tailscale but that provides mullvad exit nodes that any internet activity outside of your containers would be behind. Gluetun allows you to put your containers behind vpn as well. As far as speeds go, it might slow down abit but not much