network speed issue. been at this for days!!!

[droogiee]

ok. i cannot figure this out. i have 250/250 fiber service.
on a handful of my personal and some business windows pcs, i am having issues. i am getting 127 down, and 180 up. other pcs on the network get 250 up and down all day long.
it is a pc issue. and i cannot figure this out!!
i have tried plugging the pc with issues directly into the modem, bypassing all switches and routers, and i still get 127 down, 180 up. i have tried this with other problem pcs, same thing.
this is all wired of course.
on my problem dell laptop, i have disabled everything, tried in safe mode with networking, nothing works. strangely, on one of the problematic pcs, going into safe mode fixed it!? but comes back when logging into windows 10 normally. all the problem machines are getting the same numbers, 120-ish down, 180 ish up.
i have tried different cables, different switches, cable testers, etc. like i stated, i have even gone directly into the modem with my machine and i get the same numbers. i will use a good machine, and will get 250/250 anywhere in my network, at the modem, at the end of a long run. everywhere!
i try to think what these machines have in common, and i cannot for the life of me figure this out.
i have uninstalled all vpn and firewall applications,. i have created a new user. i have uninstalled all network stack, and reinstalled. i have QOS and all traffic shaping off on the router (and again, i have tested many times bypassing all my network infrastructure)
i am out of ideas. i ordered a gig usb/nic dongle.. curious to see what will happen when that arrives.. i am out of ideas guys!
what am i overlooking!!!??
thanks for your time
p

 

[ChatGPT]

Hi droogiee — the big clue is this:
One problem PC is fast in Safe Mode but slow in normal Windows.
That strongly points to a Windows software/driver/filter issue, not cabling, switches, router QoS, or the ISP. Since several affected PCs are landing around the same 120-ish down / 180-ish up, I’d look for something common like a NIC driver feature, OEM network optimizer, VPN/firewall filter driver, antivirus web shield, Dell/Killer/SmartByte software, or a TCP tuning setting.

First: confirm this is not a bad test method​

Before changing more settings, test with the same method on a good PC and bad PC.

1. Use the same speed test site/app on both machines.
2. Try more than one target, for example:
   • Ookla Speedtest app
   • Fast.com
   • Cloudflare speed test
3. Test in a private/incognito browser window or the Speedtest desktop app.

Expected result: if only browsers are slow but the Speedtest app is fine, look at browser extensions, antivirus HTTPS scanning, or proxy settings.

Check the negotiated link speed​

Even though your numbers are above 100 Mbps, still confirm Windows sees the adapter correctly.

1. Open PowerShell as Administrator.
2. Run:

Get-NetAdapter | Select-Object Name, Status, LinkSpeed, DriverInformation
Expected result: the wired adapter should show 1 Gbps or higher.
If it shows 100 Mbps, that is a cabling/NIC negotiation issue. If it shows 1 Gbps, keep going.

Check for third-party network filter drivers​

This is where I would focus.

1. Press Win + R.
2. Type:

ncpa.cpl

1. Right-click the Ethernet adapter.
2. Click Properties.
3. Look at the checked items.

Normal items usually include:

Client for Microsoft Networks
File and Printer Sharing
QoS Packet Scheduler
Internet Protocol Version 4
Internet Protocol Version 6
Microsoft LLDP Protocol Driver

Look for things like:

Killer
SmartByte
cFosSpeed
VPN
Fortinet
SonicWall
Cisco
OpenVPN
WireGuard
Tailscale
ZeroTier
Npcap
NetLimiter
GlassWire
ESET
Avast
AVG
Bitdefender
Kaspersky
Malwarebytes
VMware
VirtualBox
Hyper-V Extensible Virtual Switch

1. Temporarily uncheck suspicious third-party items.
2. Click OK.
3. Reboot.
4. Test speed again.

Important: Do not remove IPv4. If something breaks, go back into the adapter properties and re-check what you disabled.

Look specifically for Dell SmartByte / Killer / optimizer software​

On Dell systems, I would especially check for these:

SmartByte
SmartByte Drivers and Services
Killer Control Center
Killer Network Service
Dell Optimizer
Intel Connectivity Performance Suite

1. Open Settings > Apps > Installed apps.
2. Uninstall any Dell/Killer/network optimization tools listed above.
3. Reboot.
4. Test again.

These tools can absolutely cause weird asymmetric throughput issues.

Reset Windows TCP tuning​

Run these from Command Prompt as Administrator:

netsh winsock reset
netsh int ip reset
netsh int tcp set heuristics disabled
netsh int tcp set global autotuninglevel=normal
netsh int tcp set global rss=enabled
netsh int tcp set global ecncapability=disabled
ipconfig /flushdns

Then reboot.
After reboot, check current TCP settings:
netsh int tcp show global
You generally want:

Receive Window Auto-Tuning Level : normal
Receive-Side Scaling State       : enabled
ECN Capability                   : disabled

Disable problematic NIC offload/green features​

Some Realtek, Intel, and Killer adapters behave badly with offload features.

1. Open Device Manager.
2. Expand Network adapters.
3. Right-click the Ethernet adapter.
4. Click Properties.
5. Open the Advanced tab.
6. Temporarily disable these if present:

Energy Efficient Ethernet
Green Ethernet
Large Send Offload IPv4
Large Send Offload IPv6
Receive Segment Coalescing
Interrupt Moderation
Flow Control

1. Leave Speed & Duplex on:

Auto Negotiation

1. Reboot and retest.

If performance gets worse, undo the changes one by one.

Try a clean boot​

Since Safe Mode helped one PC, do this next.

1. Press Win + R.
2. Type:

msconfig

1. Go to Services.
2. Check Hide all Microsoft services.
3. Click Disable all.
4. Open Task Manager > Startup apps.
5. Disable all startup items.
6. Reboot.
7. Run the speed test.

Expected result: if speeds return to 250/250, re-enable services/startup items in small groups until the bad one appears.

Compare a good PC and bad PC​

On one good PC and one bad PC, run:
Get-NetAdapterBinding -Name "*" | Sort-Object DisplayName | Format-Table Name, DisplayName, Enabled -Auto
Also run:
Get-NetAdapterAdvancedProperty | Sort-Object DisplayName | Format-Table Name, DisplayName, DisplayValue -Auto
You are looking for differences in:

Binding/filter drivers
Offload settings
Energy Efficient Ethernet
Speed/Duplex
Vendor utilities
VPN remnants
Security software drivers

Best isolation test: LAN iperf​

Internet speed tests can mislead. Test PC-to-PC inside your network.

1. On a known-good wired PC, download/run iperf3.
2. Start server mode:

iperf3.exe -s

1. On the bad PC, run:

iperf3.exe -c GOOD_PC_IP
Expected result: on gigabit wired LAN, you should usually see hundreds of Mbps, often 900+ Mbps.
If LAN iperf is also poor, the issue is definitely local to that Windows install/NIC/driver stack. If LAN iperf is fast but internet speed test is slow, look harder at browser/security/proxy/TCP settings.

Your USB gigabit NIC test will be useful​

When the USB NIC arrives:

1. Disable the onboard Ethernet adapter.
2. Plug in the USB NIC.
3. Let Windows install the driver.
4. Test speed.

Interpretation:

• USB NIC gets 250/250: likely onboard NIC driver, NIC advanced property, Dell/Killer/Realtek software, or hardware issue.
• USB NIC also gets 127/180: likely Windows network stack, filter driver, security/VPN software, optimizer service, or TCP setting.

My top suspects​

Based on your symptoms, I’d check in this order:

1. SmartByte / Killer Control Center / Dell Optimizer
2. Antivirus web shield or HTTPS scanning
3. Leftover VPN/firewall filter driver
4. NIC offload or Energy Efficient Ethernet setting
5. Broken TCP auto-tuning setting
6. Bad OEM NIC driver from Windows Update

If you can, post the output of these from one good PC and one bad PC:

Get-NetAdapter | Select Name, Status, LinkSpeed, InterfaceDescription
Get-NetAdapterBinding -Name "*" | Sort DisplayName | Format-Table Name, DisplayName, Enabled -Auto
netsh int tcp show global

That comparison should make the common cause much easier to spot.

 

[droogiee]

OH MY GOSH!!! this did it!!

netsh winsock reset
netsh int ip reset
netsh int tcp set heuristics disabled
netsh int tcp set global autotuninglevel=normal
netsh int tcp set global rss=enabled
netsh int tcp set global ecncapability=disabled
ipconfig /flushdns

what did that do exactly!!!!???
i have been pulling my hair out for 2 days!!!! oh my goodness!
thank you thank you thank you!
what did that do? how did that get tweaked??

 

[ChatGPT]

Great! Glad that fixed it, droogiee. That basically confirms the issue was inside the Windows networking stack, not your fiber, modem, router, switches, or cabling.

What those commands did​

netsh winsock reset​

This reset the Winsock catalog.
Winsock is the Windows layer that apps use to talk to the network. VPNs, firewalls, antivirus web filters, packet capture tools, proxy tools, and “network optimizer” utilities can insert hooks/providers into Winsock.
If one of those entries gets broken or badly tuned, you can get exactly what you saw:

Same PC is slow everywhere
Other PCs are fine
Safe Mode may be faster
Download/upload are weirdly asymmetric

This command removes/repairs those custom Winsock entries and puts Windows back to a clean default state.

---

netsh int ip reset​

This reset the TCP/IP stack.
It rewrites a bunch of Windows networking registry/configuration defaults for IPv4/IPv6 interfaces. Think of it as “rebuild the core Windows IP plumbing.”
It can fix bad settings left behind by:

VPN clients
Firewall suites
Old NIC drivers
Network tuning tools
Malware/adware proxy tools
Broken Windows updates
Manual registry tweaks
Corporate/management software

---

netsh int tcp set heuristics disabled​

This disabled Windows TCP heuristics.
TCP heuristics can automatically alter TCP behavior based on what Windows thinks is best. Sometimes it decides poorly or remembers a bad state.
In your case, this may have been preventing Windows from using the right receive window behavior.

---

netsh int tcp set global autotuninglevel=normal​

This is probably the big one.
This restored TCP Receive Window Auto-Tuning to normal.
If auto-tuning is disabled, restricted, or corrupted, downloads can cap out strangely, especially on higher-speed connections. Your 127 Mbps down / 180 Mbps up pattern fits that kind of issue.
Auto-tuning lets Windows dynamically scale the TCP receive window so the connection can actually fill the available bandwidth.
If it had been set to this:

disabled
restricted
highlyrestricted

Then your download side could be artificially limited.

---

netsh int tcp set global rss=enabled​

This re-enabled Receive Side Scaling.
RSS lets Windows spread network receive processing across multiple CPU cores instead of one core doing all the packet handling.
On modern wired connections, RSS should generally be enabled. If it was off, a machine can bottleneck on packet processing even when the NIC link is fine.

---

netsh int tcp set global ecncapability=disabled​

This disabled ECN, Explicit Congestion Notification.
ECN is not bad by itself, but some routers, firewalls, ISP gear, or middleboxes handle it poorly. If ECN negotiation gets weird, throughput can suffer.
Leaving it disabled is common and safe for normal desktop/business use.

---

ipconfig /flushdns​

This cleared the local DNS cache.
This probably was not the main fix for your speed issue. It just removes cached name lookups. Helpful, but not likely responsible for restoring bandwidth.

So what was actually broken?​

Most likely one of these:

TCP auto-tuning was disabled or restricted
Winsock had a bad filter/provider from old software
A VPN/firewall/AV left behind a network hook
A “network optimizer” changed TCP settings
Dell/Killer/SmartByte/optimizer software modified the stack
RSS or TCP heuristics were in a bad state

Given that Safe Mode fixed one of the PCs, I’d still strongly suspect something that was installed on the affected machines at some point.

How did it get tweaked?​

Usually this happens from one of these:

VPN clients
Third-party antivirus/firewall suites
Dell SmartByte
Killer Control Center
Network optimization utilities
Old “speed up internet” tweak scripts
Driver update tools
Business endpoint/security software
Remote access tools
Packet capture/filter tools

Even after uninstalling a VPN/firewall, the low-level network filter can sometimes leave settings behind. That is why uninstalling the app does not always undo the damage.

Check what changed now​

Run this on one of the fixed PCs:
netsh int tcp show global
You want to see roughly:

Receive-Side Scaling State          : enabled
Receive Window Auto-Tuning Level    : normal
ECN Capability                      : disabled

If those stay that way after reboot, you’re good.

If it comes back​

If the problem returns, something is actively changing the settings again.

• Check installed apps for:

SmartByte
Killer Control Center
Dell Optimizer
cFosSpeed
NetLimiter
GlassWire
VPN clients
Third-party firewall/AV web shield
Network optimizer tools

• Run this again to see the TCP state:

netsh int tcp show global

• Compare before/after installing updates or rebooting.
• If it changes back, that tells us a service, driver, policy, or app is reapplying the bad setting.

Bottom line​

You didn’t fix a cable problem — you reset Windows’ software networking path back to sane defaults.
Your connection was physically fine. Windows was probably limiting or mishandling TCP receive behavior, and the reset restored normal throughput.
Very happy that got it! That kind of issue is maddening because every physical network test looks “right” while Windows is quietly sabotaging itself.