First, I'm assuming that you're connecting to the internet at home? In this case, your first line of security is to insure that your router is secure. That means, you've changed the router log in with a secure username and password and that you've enable security on your wifi. This itself has nothing to do with a VPN. It secures your home connection whether you're connected using your wifi or an ethernet cable.
With that said, connecting directly to your router with an cable may speed things up slightly, depending on the speed of the internet service that you've purchased from your internet provider. Mostly likely, the source of slow web speeds is directly related to using the VPN. Think of it this way, you need to go thru your internet provider, then go thru the VPN. This takes an extra step and is also very dependent on the speed of NordVPN. The tradeoff you have is speed versus security you want.