Recently upgraded our home network with a new modem and router. Haven't check speeds in awhile but decided to after installing the new equipment. Me and my wife's computer sit pretty much side by side wired to our new Orbi router. Ran a speed test on both PC's, with hers getting far better results. Computers are pretty similar, but I have a few upgrades like graphics card and more ram but all-in-all, they are both high-end computer gaming rigs. However, after I ran the tests, her PC was getting a rocksteady 580-600 mbps download speed while mine was starting off near the same, but over the coarse of the test, slowed pretty consistently to around 300 mbps - sometimes higher/lower, but roughly a 200 mbps loss in speed from the one sitting right next to me. What I noticed was the initial tests were maintaining a pretty good speed, but after about 8-10 seconds, it drastically went down hill. I've scoured the interwebs for potential fixes but most advice was outdated or offered general tips on how to improve wireless speeds. Thus, I land here in hopes the mighty brains of networking might help steer me to a solution, or at the very least an explanation of what is going on. So without further ado, here's the shortlist of things I've done prior to coming here seeking help. I'm fairly savvy with computers, but I fully admit, networking is pretty foreign to me.

Rebooted all affected equipment several times: modems, routers, pc's.
Tried swapping the ethernet cords between PC's to see if the cord was bad or the cause of the slowdown.
Swapped ports on the router.
Watched the Task Manager like a hawk to see if anything was hogging my network bandwidth.
Checked various setting inside the router to see if any bright neon signs or red flags were on display.
Flushed the DNS cache.

The newly purchased equipment involved (although I cannot say the problem didn't exist prior to replacing):
Netgear Nighthawk CM 2000 Modem
Netgear Orbi Mesh WiFi 6 (RBK 753s)
ISP is Comcast

Thanks for any help on this matter. And if any other specs or technical data is needed please let me know and I'll do my best.