Extremely slow internet download speeds

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Level 1

4 points

Extremely slow internet download speeds

Hello all

My iMac has vastly reduced Internet download speeds. The precise issue is that download speeds are as expected for roughly 30 minutes when the computer is powered on and then drop to unacceptably low speeds (less than 1Mbps!).

From my perspective this happened out of the blue without any changes being made by me to my system (computer or network).

Brief background:

I reset my router, including a factory reset, and my Mac's SMC. Helped for about 30 minutes.
I contacted my ISP (Plusnet UK) who, after a very brief conversation, decided to post me a new router as the one I had was quite old. I am now running this new router but the problem remains.
It was possible to split into 2.5GHz and 5GHz with my old router and I would achieve download speeds of around 30Mbps and 60Mbps respectively. Both fine for my needs.
The new router has a better range (meaning devices in the far reaches of my stone walled cottage now receive steady access to the network) BUT it isn't possible to split the signal. That said, although 5GHz would give me faster speeds, it would not solve my problem, given how much my download speed reduces.
I have reset the NVRAM. Helped for about 30 minutes.
I uninstalled my VPN, although the application was not running. No difference made.
Altered DNS settings to Google DNS. Helped for about 30 minutes.
Other devices such as my work Windows laptop, iPhone, Sonos speakers and TV all connect to the network with expected performance/download speeds.

Any advice greatly appreciated. I also realise my post is probably lacking some info you'll need to identify the issue, so please let me know what else I need to supply.

Thanks in advance!

TK

iMac 27″, macOS 15.6

Posted on Sep 18, 2025 3:59 AM

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Answer 1

Top-ranking reply

MrHoffman

Level 10

149,353 points

Sep 18, 2025 7:00 AM in response to Backhander

On the slight possibility that bad hardware is involved, ensure you have complete and current backups.

Two sets of data: the first is the hardware and software inventory, and the second is a Wi-Fi overview.

Please download and run (free) EtreCheck, and share the results to the clipboard. Then open a new reply here, and press the Additional Text button that looks like a printed page, and paste and post the hardware and software configuration report here. From that, we can get a little more detail about what is installed and what is running here.

On Mac, the basic Wi-Fi environment characteristics are visible when using Option-Click on the Wi-Fi logo 🛜 in the menu bar to capture some Wi-Fi network data.

For us to look at the Wi-Fi network data here, post the items shown in the following image including the Security mode and from Channel to NSS inclusive (Security, Channel, Country, RSSI, Noise, Tx Rate, PHY mode, MCS, NSS, green highlight) (without posting the Wi-Fi address, any local HotSpots that might be present, the network name, or the BSSID, all of which are expurgated from the image shown below, red highlight), and we can take a look at the basic network environment.

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Answer 2

den.thed

Level 10

92,764 points

Sep 18, 2025 6:56 AM in response to Backhander

Your profile indicates that you are running macOS 15.6.

If that is correct, then you should update the iMac to macOS 15.7.

see > What's new in the updates for macOS Sequoia - Apple Support

What VPN were you using and how did you uninstall it.

Even if you followed the VPN developers uninstall instructions.

The VPN could have made some system changes that are still in effect.

So in addition to what you've tried so far.

Startup the iMac in Safe Mode and test.

Then restart normally and re-test.

see > Start up your Mac in safe mode - Apple Support

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Answer 3

Sep 18, 2025 6:47 AM in response to Backhander

Please run Etrecheck and post its full report here. Use the "additional text" button and paste the report into the text box.

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Answer 4

MrHoffman

Level 10

149,353 points

Sep 24, 2025 6:43 AM in response to Backhander

A few floors is likely the issue.

You’re likely at the edge of the range.

Or variously, past.

I’m using WiFi Explorer app to get a better representation of the signals.

Your ISP may be able to provide a mesh setup for better coverage.

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Answer 5

Backhander Author

Level 1

4 points

Sep 23, 2025 4:42 AM in response to Backhander

Hello, thanks all or your replies and apologies for the delayed response. I have updated to Tahoe 26.0 and thought it had fixed the issue but it has not.

I was running Mulvad VPN and installed by removing from applications folder and deleting from trash - which might not have been ideal? That said, I hadn't run the application for a few weeks before I experienced this issue.

I can start in safe mode but what am I looking out for when I do?

Wifi info below. Will this help diagnose the issue without me having to install EtreCheck?

[Edited by Moderator]

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Answer 6

MrHoffman

Level 10

149,353 points

Sep 23, 2025 7:11 AM in response to Backhander

Your signal strength is fair, your SNR is good (due entirely to an unusually low noise floor), and 20 MHz slots in 2.4 GHz is a decidedly not-optimal choice.

How old is this router?

Does it have 5 GHz?

Check the results again when you have issues, too. I’d expect this Wi-Fi to fall apart if the noise comes up to something closer to the values I see elsewhere.

The VPN vendor has removal instructions, if it is more than dragging it to the trash and removing any related profiles.

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Answer 7

Backhander Author

Level 1

4 points

Sep 24, 2025 6:26 AM in response to MrHoffman

Thanks for your response.

Those results were provided whilst I was experiencing issues.

The router is new (my ISP posted it to me a couple of weeks ago). Of course, the device might be new but I can't speak for the technology.

I can't control whether my Mac connects to the 2.5 or 5GHz signal. I suspect it connects to the former as my Mac is a few floors above where the router is located.

Can anyone please elaborate on the safe mode test that has been suggested to me?

Thanks again.

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