I have a new, powerful PC, but my internet speed is only 40 Mbps instead of the 1000 Mbps my plan provides. My old laptop, using the same cable, delivers 800 Mbps.
A new Cat6 cable, a router reboot, a network reset via the command line, and a DNS flush didn’t help. No traffic is being used in the background. What could be the cause?
It sounds like a classic bit/byte issue, but 40 Mbps on a gigabit link is too much of a drop even for this kind of confusion. Since the laptop is only pushing 800, we need to isolate the problem: is it the OS or the hardware?
Fix checklist:
Power saving: Windows loves to throttle your network card to save power. Go to Device Manager, open the adapter’s properties, and completely disable Green Ethernet, Energy Efficient Ethernet, and all other checkboxes in power management.
Bludware: All those network “optimizers” from vendors like Dragon Center or GameFirst are pure scams. Their “packet prioritization” actually cuts throughput. Delete that software
Are you sure you’re really using a Cat6 cable and that Windows hasn’t switched to a bad Wi-Fi connection? I’ve seen a million times where someone connects to Ethernet, but the computer stays on the 2.4 GHz band because it saved the password. If you’re using Wi-Fi, the 2.4 GHz band is currently a saturated band. You should download NetSpot to your laptop or phone and see what’s going on in inspector mode. And it wouldn’t hurt to run a survey. Look at the heat map of signal strength and, more importantly, interference levels. If you see a lot of overlapping channels, that’s the cause. NetSpot is a useful tool for detecting “dead zones” or understanding what’s happening in the radio air. Move your router or switch to the 5 GHz/6 GHz band and see if that solves the latency issue.
Before buying hardware, run a Linux Live USB test. Boot into Ubuntu/Mint from a flash drive. If the speed soars to 900+ Mbps, your hardware is perfect, and the problem is purely software-related in Windows. In Microsoft OS, the TCP Window Auto-Tuning algorithm often fails. Due to bugs, it permanently locks the Receive Window (RWIN) size to the minimum, severely limiting throughput regardless of the link. If the Linux test is successful, you should debug the registry using regedit or perform a clean reinstall of Windows. Also, check the physical connectivity. Make sure you haven’t plugged the patch cord into a low-speed pass-through port (for example, the LAN port on a VoIP phone) that is hardware-locked to 100BASE-TX (Fast Ethernet).
Since you mentioned the laptop works fine right next to the PC, it’s highly likely local interference or a bad configuration. If you’re using Wi-Fi on the desktop, check your antennas. A lot of people forget to actually screw in the external antennas that come with the motherboard, assuming the internal headers are enough. They aren’t. If you want to be scientific about it, use NetSpot to check the signal-to-noise ratio (SNR) right where your PC sits. If the SNR is low, your PC is spending all its time re-sending dropped packets instead of actually downloading data. If the signal is clean and you’re still slow, look into your ISP’s provisioning. Sometimes they “bind” a certain speed to a specific MAC address, though that’s rarer these days. Also, check if your router has “Quality of Service” (QoS) turned on. If the router thinks your PC is a low-priority device, it’ll give all the juice to your laptop or TV instead.