Really slow Internet speeds (Macbook Pro Mid-2012) with High Sierra & Mojave OSs

My wife and I use Cox Data service with a Cox-supplied Arris Panoramic modem/router. Together, they're capable of Internet connections with speeds in excess of 300 Mbps (download). My wife's obsolescent 2010 iMac, running High Sierra 10.3.6 and connected by Ethernet, is currently capable of 180 Mbps down and reasonable numbers up.

My built-up Macbook Pro (Mid-2012, 13", 16 GB RAM/500+ GB SSID), also running High Sierra and using an Ethernet connection, was achieving DL speeds of 325-350 Mbps. Speeds via wifi (5 GHz) were about 35-40% slower, as one might expect; but my NordVPN was sailing. Pings for each were in the low double-digits.

However, beginning with the latest version of High Sierra OS (10.13.6) and continuing with Mojave OS (10.4), my net speeds have slowed dramatically. Pings are larger. My download speeds via Ethernet have dropped by 75%, to about 75-80 Mbps. Via wifi, that number is about 55-70 Mbps. Both are crummy for the price we pay.

I checked the modem, rebooted it, worked up new DNS numbers, etc. There's nothing wrong with the modem. And my Ethernet cable is CAT 5e, high-capacity. As stated earlier, there's nothing wrong with my wife's iMac's speed. So I believe something's wrong with my Macbook Pro. I've been running all sorts of tests including Network Diagnostics and EtraCheck, but no software or Settings issues have been reported.

Is it possible that a hardware component, a communications card, has gone bad? Is there a way to test for it? If so, what's the card's name -- and if you've tested and replaced yours, how easy is it to do so? Thanks for your advice.

For too many folks in America, my complaining about tooting along at "only" 80 Mbps must seem like crying crocodile tears, when they get only half that at best (a result of failed national policies). But I make my livelihood working on my Mac and that includes sharing video, holding real-time meetings, and streaming recordings -- all of which are best experienced at higher speeds. So it's important for me to fix what I can.

Thanks for your advice and suggestions.