Extreme Packet Loss and Latency Issues with New PC Build

Hey everyone, I'm really struggling with serious packet loss and latency while gaming on my newly built PC. I've gone through almost every troubleshooting step I can think of and could really use some guidance. Here's what I've got:

I just finished assembling my new PC, and after installing all the necessary drivers, I jumped into some games and immediately noticed the extreme packet loss and lag. It's not limited to just one game, as I've tried multiple titles and the issue persists.

When I run speed tests, my internet speeds are normal, so I'm scratching my head over why I'm experiencing this while gaming.

Here's what I've already done to try and fix this:
- Swapped out ethernet cables and even tried using WiFi, but to no avail.
- Confirmed that my ISP is not the issue.
- Tested my old gaming PC with the same setup and encountered no packet loss or latency.
- Swapped both the CPU and motherboard (same models).
- Updated my drivers and also rolled back to older versions for testing.
- Did a clean install of Windows and removed unnecessary software from ASUS and Windows.
- Tweaked my internet settings in Windows, fiddled with DNS, buffer rates, and various optimization tips I found online.
- Checked BIOS settings for any mismatched configurations.

I'm quite confident the problem isn't hardware-related since my old PC works perfectly, which makes me think it could be a software issue that I'm unable to identify. The last hardware change I haven't made is switching out the GPU, but I'm not sure how that could affect packet loss.

I've heard some talk about my motherboard's Realtek ports potentially causing issues, while some say the Intel ports are better, but I haven't been able to verify this. I've ordered a PCIe Ethernet card and will be installing it soon as a last resort. If that doesn't resolve the issue, I'm at a complete loss. Any insights or suggestions would be greatly appreciated!