I'm really at my wit's end here. I've been troubleshooting my new PC build for over 6 hours, and I can't figure out why my Ethernet keeps maxing out at 100 Mb/s, and my Wi-Fi is barely hitting 120 Mb/s. All my other devices (laptop, other PC, phone) are cruising along with speeds between 400-600+ Mb/s on Wi-Fi and 650-700+ Mb/s on Ethernet using the same cables and modem port.
Here's what I've tried:
- Swapped out three brand-new Cat-6 cables
- Plugged directly into the modem (which is a Spectrum EN2251 DOCSIS 3.1)
- Power-cycled the modem
- Updated the BIOS, tried flashing down and then back up
- Cleared CMOS and discharged the system
- Fresh installs of Windows 11 and Pop!_OS, both on metal
- Tried every version of the Realtek drivers I could find
- Disabled Green Ethernet / EEE / Auto Disable Gigabit
- Forced settings to 1.0 Gbps Full Duplex
- Used different ports and devices; every other device gets full speeds
Here are my specs:
- AMD Ryzen 9 9900X3D
- MSI MAG X870E Tomahawk WiFi (Realtek RTL8125B 2.5Gb Ethernet, Qualcomm FastConnect 7800 Wi-Fi 7)
- 64GB G.Skill Trident Z5 Neo DDR5-6000
- Gigabyte RTX 5080 Aorus Master
The Realtek NIC just won't negotiate over 100 Mb/s, even with brand-new cables. Wi-Fi isn't much better either. I definitely feel discouraged since this is supposed to be my dream machine, and it's not performing. Before I consider sending this back, does anyone have any ideas? Is there a known issue with the RTL8125B? Should I look into hardware defects, or is there something I'm missing?
5 Answers
Have you tried updating your chipset drivers? Also, since it’s an MSI board, make sure you’re not using the Killer network software—it can cause issues. Just check your Device Manager and ensure everything is up to date.
It sounds like your NIC is the issue here. If it won’t link at gigabit speeds while other devices do just fine on the same setup, it’s probably a bad network interface card. I’d suggest just getting a PCIe NIC for around $30. It’ll save you the hassle of RMA-ing the whole motherboard, and you can get back to enjoying your new build without further delay.
In my case, I was stuck with bad speeds until I replaced my motherboard. I got an Asrock B850 Riptide WiFi and it fixed everything for me. If you’ve seriously tried everything and still see no improvement, it might just be worth it to swap out the board if that’s feasible.
Double check your NIC's settings! Make sure it's set to "Auto Negotiation" for the best possible speed. You should also confirm that it’s not limited to 2.4 GHz—check that in the properties under the 'Advanced' tab. If none of that works, consider getting a new NIC card or a USB Wi-Fi dongle just to test it out.
Interesting! I had the same problem with my setup. The Ethernet link showed 100/100 when plugged in directly, but when I switched to a USB-C to Ethernet dongle, it suddenly shot up to 1000/1000! Might be worth exploring that route for a quick fix!
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