X870E Crosshair Hero Realtek 5g Ethernet being throttled.

[ski_eternal]

Been racking my brain trying to figure out why the Realtek 5g is being throttled to 30mbps or less while the Intel 2.5g runs buttery smooth to it's isp rated speeds of 1000mbps. My setup is as follows:
995X3D + Crosshair Hero x870e + RTX 3090 + Unifi Dream Router + 1200w SuperFlower PSU + 96GB G.Skill RAM

Here's everything I've tried:

• Downloaded the latest Realtek PCIe 5 GbE driver directly from Realtek’s official site And ASUS.
• Same ethernet cable works fine on the Intel Lan Controller, so shouldn't be an issue on Realtek right?
• Uninstalled completely from device manager and ran default Windows 11 drivers.
• Installed the driver manually (after verifying INF match) to ensure proper RTL8126 support.
• Tested for network connectivity, link stability, and 5 Gb throughput under load.
• Confirmed Windows 11 x64, BIOS, chipset, and LAN firmware are fully up-to-date.

Monitored performance and system stability post-install to verify no driver conflicts

Toggled these settings on and off:

Setting Recommended value

Energy Efficient Ethernet / Green Ethernet	Disabled
Power Saving Mode / Smart Power Down	Disabled
Flow Control	Disabled (test both later)
Interrupt Moderation	Disabled
Large Send Offload v2 (IPv4/IPv6)	Disabled
Receive Side Scaling	Enabled
Jumbo Frame	Disabled / 1500 bytes

I'm sure there are other things I neglected to mention which I have already tried but I've given up at this point because everything else on this board functions smoothly except Realtek adapter. So I'm all ears if anyone has a solution.

[ExcessiveGBH]

Is this a new build?

Check ethernet port on MB, inspect contacts. 

Windows version 24H2, 25H2? 

Had an issue with a clean install of 25H2 that my Samba network drive was locked at 35MB for copy & paste.  Should be 80 - 100MB.

Maybe have a look at this fix, this may not be relevant to your issue, but you never know. This sorted my issue.

Windows 11 24H2 - Network limited to 25MB/s (internal and external) : r/WindowsHelp

Never assume in tech world, just because it works in here that also good for this. 

[ski_eternal]

Yes it's a new build. My fear is it's a hardware issue since the board has two ethernet controllers and one works just fine, while the other works leads me to believe it's the Realtek as the issue. But I'll report back with my results once I tested it.

[ski_eternal]

Unfortunately, it didn't help much, i got speeds to hit 200mbps but it caps it there. Whereas if I un-plug it and re-plug it back to the Intel 2.5gbe controller it goes back to 1gpbs.

[ExcessiveGBH]

It's looking like the network port on MB, could be a dry joint.

Not sure how your internet works in your country, but in Australia you can bypass your router and plug ethernet from PC straight into NBN box. Use this to bypass router just to eliminate bad firmware.