Assuming you’re talking about adding four Ethernet to your PC, unless you have really long cable lengths (25+m) back to your router or have already filled those ports then the only real advantage is that it’s likely to be more convenient to attach devices near your PC rather than have to provide extra power outlets and physical space for extra devices near your router, bearing in mind that could also cause interference to your wifi unless they’re at least 1m from the router. As for speed issues, if your other devices are receiving faster connections AT THE SAME TIME either your PC’s storage (hard disk) can’t keep up (but you said it’s been better previously) or it is connecting to your 2.4GHz network instead of the 5GHz. Note that if a PC or laptop is connected on wifi, it doesn’t drop that automatically in favour of Ethernet LAN when you plug in, despite what the system tray icon implies. You need to disable wifi to be sure it’s using Ethernet. Other possibilities include some new equipment somewhere near your cable run causing interference that wasn’t there last time you tested. Would be useful if you described your network setup, i.e. what’s plugged into what, and what else is connected over wifi. Wifi and LAN will operate independently - an old, slow device on wifi won’t impact your Ethernet LAN speeds, and if both ends of the connection are into Gigabit ports with a green light showing not yellow, speed should not be lacking.
Just occurred to me - I’d choose to buy a stand-alone multiport hub/switch over adding a card to your PC - if you connect everything into your PC it will have to be left on for you to access your NAS from other devices on your network. I have a couple of WD Cloud NAS boxes, plugged into an 8-port Netgear Gigabit hub, and I can see the content from phones, TV, and laptops. If I’d plugged them directly into a PC, it would have to be on 24/7 to support that, using a lot more electricity than the hub does.
Sorry to hear if you have had issues with your internet connection. How are things looking at the moment? Is this only happening with your PC? Have you tried using different PC or Laptop to check if there's any difference?