Solved Realtek PCIe 2.5GbE network adapter - not reaching its nominal speed


bob2026

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Hi, when I found out my modem allowed speed up to 4 Gbs I purchased a Realtek 2.5Gbs adapter. When I plugged it into the motherboard it appeared to be actually running full speed, at least according to W11/Settings/Network&Internet. The light on the Ethernet switch was orange, all the others green. I found that this was proof of a higher speed. Several months later I noticed it was on 1Gbs again, like the previous adapter. I changed the speed manually in the Device Manager, but it does not register. I think I have the latest driver (from 02.02.2026). Any idea how to push it to its nominal speed, please ...? (it had already slowed down before yesterdays automatic update to a newer insider preview version.)
 
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EDIT: For clarification, when I say that I have not heard of a device rated for 40Gbps, I mean specifically Ethernet devices and cables.

First, I have never heard of any device rated for a 4Gb per second speed. I have heard of 1 Gb, 2.5 Gb, 5 Gb, and 10 Gb, but never 4. I am not saying that you are wrong just that I have never heard of it so it might be worth something to double check what the interface speed really is of your router.

Secondly you may just want to make sure that you are plugged into the proper port on your router. Some routers have only certain ports that are high speed and other ports that are slower. Your system should be automatically negotiating the highest possible speed with the router so if that port is truly capable of at least 2.5 Gb, then the only other thing that I could think of might be that the cable you are using is not handling that speed properly.
 
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Hi, when I found out my modem allowed speed up to 4 Gbs I purchased a Realtek 2.5Gbs adapter. When I plugged it into the motherboard it appeared to be actually running full speed, at least according to W11/Settings/Network&Internet. The light on the Ethernet switch was orange, all the others green. I found that this was proof of a higher speed. Several months later I noticed it was on 1Gbs again, like the previous adapter. I changed the speed manually in the Device Manager, but it does not register. I think I have the latest driver (from 02.02.2026). Any idea how to push it to its nominal speed, please ...? (it had already slowed down before yesterdays automatic update to a newer insider preview version.)
I had a similar issue when I bought a TP-Link UE302C USB C to Ethernet adapter for my PC. It wasn't reaching its full download speed potential and I couldn't work out why. Eventually I discovered that by going into the Network Ethernet change settings for the Realtek 2.5 Gb E Family Controller, selecting properties, Configure, Advanced and scrolling down to Speed and Duplex, there's a full drop down list of different speed settings. I chose 2.5 Gbps Full Duplex and it solved the issue.
 

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scrolling down to Speed and Duplex, there's a full drop down list of different speed settings. I chose 2.5 Gbps Full Duplex.
See above : "I changed the speed manually in the Device Manager."
 

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First, I have never heard of any device rated for a 4Gb per second speed. I have heard of 1 Gb, 2.5 Gb, 5 Gb, and 10 Gb, but never 4. I am not saying that you are wrong just that I have never heard of it so it might be worth something to double check what the interface speed really is of your router.

Secondly you may just want to make sure that you are plugged into the proper port on your router. Some routers have only certain ports that are high speed and other ports that are slower. Your system should be automatically negotiating the highest possible speed with the router so if that port is truly capable of at least 2.5 Gb, then the only other thing that I could think of might be that the cable you are using is not handling that speed properly.
Thanks. Isn't it strange that it delivered immediately 2.5Gb on plug in? My router can deliver up to 8Gb (checked my contract). All its ports deliver 8Gb, my Ethernet switch delivers 2.5Gb and the cable I bought can handle up to 40Gb.
 

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the cable I bought can handle up to 40Gb

Are you sure that you are looking at the specs for the right thing? This sounds like a fictitious number. As far as I am aware, there is no such thing as 40Gbps over Ethernet, only fiber. However, there are USB standards that support 40Gbps. USB 4 can support 40Gbps (often only implemented as 20Gbps), but Thunderbolt 3, 4, and USB 4 version 2 all support 40Gbps. But nothing over Ethernet does, at least not that I have ever heard.
 

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But nothing over Ethernet does, at least not that I have ever heard.
I am talking about the cable ONLY. You wrote: "... the cable you are using is not handling that speed properly." The cable is not the problem. Here are its specs: Goobay's slim-format Category 8.1 network cable is ultra-flexible, features U/FTP shielding and provides fast, reliable network connections. Its ultra-slim and flexible design makes it easy to install in confined spaces, making it ideal for use in a server or network environment. With a bandwidth of 2000 MHz, it supports speeds of up to 40 Gbit/s for viable deployment over time.
 

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I am talking about the cable ONLY. You wrote: "... the cable you are using is not handling that speed properly." The cable is not the problem. Here are its specs: Goobay's slim-format Category 8.1 network cable is ultra-flexible, features U/FTP shielding and provides fast, reliable network connections. Its ultra-slim and flexible design makes it easy to install in confined spaces, making it ideal for use in a server or network environment. With a bandwidth of 2000 MHz, it supports speeds of up to 40 Gbit/s for viable deployment over time.
Ok, then I stand corrected. Maybe the cable is capable of that kind of speed. But what I was trying to say when I said that the cable may not be handling the speed properly is that the cable may be defective. I have come across several cables that didn't work reliably on a 2.5Gb Ethernet network even though they were Cat 7 cables. So, it still might be worth just trying another cable.

In fact, I recall on instance that I posted about here in the forum where I was getting silent corruption of data on my 2.5Gb network and it all came down to one flaky cable.
 

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I was getting silent corruption of data on my 2.5Gb network and it all came down to one flaky cable.
Well, the cable is brand new. I was not aware that a cable can work with reduced speed, I would assume it either works or doesn't, But you're right: the only way to work towards the source of the issue is to eliminate all material possibilities - even unlikely ones. I'll let you know the outcome with a different (new) cable (that could also be defective ...)
 

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Well, the cable is brand new. I was not aware that a cable can work with reduced speed, I would assume it either works or doesn't, But you're right: the only way to work towards the source of the issue is to eliminate all material possibilities - even unlikely ones. I'll let you know the outcome with a different (new) cable (that could also be defective ...)
Every time I have ever had a bad cable (and this happens with fair regularity for me) it has always been a new cable. I have yet to have a reliable cable go bad other than one or two that got violently manhandled
:-)
 

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I changed the cable, first from the router to the switch, then from the switch to the desktop. Same value: 1Mbs. But I found 2 Realtek network devices in the Device Manager, only 1 being for 2.5Gbs. When deactivating the one without 2.5 I lose my internet connection.
1785425796377.webp

Also, there are 2 wired networks, one is connected, the other isn't
1785427278419.webp

That seems to be the issue. I don't know what to do. Unmount the Realtek adapter from the motherboard and reinstall it?
 
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That seems to be the issue. I don't know what to do. Unmount the Realtek adapter from the motherboard and reinstall it?
Have you checked the BIOS settings? Usually, you can disable MB ports there.
 

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Thanks, not sure what to look for ... What are "MB ports"?
MB → motherboard

Since no one mentioned this being an OS issue, try a live boot with a Linux distro. If you cannot achieve the ~2.5Gbps speed there then the issue lies with the adapter itself.
 

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Pb solved. Opening my PC I found the network card's connection shaky. Plugged it in correctly and taped it. Now all good, back to 2.5 Gbs. Sorry for the hassle and a thousand thanks to all the helpful souls who showed mercy.
 

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