macOS EXTREMELY SLOW update download speed

Unfortunately, it's a well-known fact that ever since Apple started using its own CDN to deliver software updates, the download performance can be crippling and abysmally slow. It has nothing to do with your system. The issue is on Apple's side. You can be lucky and get a fast download, at other times you will have to wait 12 hours for a 12 GB file to finish.

[Edited by Moderator]

What internet speeds do you have?

Are you connected via WiFi or Ethernet?

How much free space do you have on your boot drive?

WiFi is much slower than Ethernet. Connect via ethernet and try again. Also try the following:

#1 - boot into Safe Mode according to Start up your Mac in safe mode - Apple Support.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

#2 - go to the App Store, search for and download the full Big Sur installer.

#3 - run the installer from the Safe Mode.

I've had better luck goring thru the App store than thru the System/Software Update preference pane.

Restart the Router and Shutdown the computer. Move computer as close the Router as possible. Disconnect ALL Other devices using the Wifi - what the Max Bandwidth to this computer. Restart Router. Pause and Hold Power Button / Touch ID for ten seconds - if am correct - the M1 should bring up Options and choose Safe Mode - load slowly - normal. Once logged in Open the Apple Store and find Big Sur and download it. This will be the Full Version 11.3.1. It is going to take some time so be patient. Once successful there should be a New Application in Applications folder called Install Big Sur. Run this application and it will proceed to update. There will be several automatic reboots along the way until Setup Assist presents itself.

Have had more success this way than the iffy Software Update method.

It happens with Adobe and Microsoft downloads, too, on 2 new computers running Big Sur. (2017 iMac Pro & 2020 M1 MacBook Air) It starts fast but gets slower and slower until it stops.

Download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Copy the report

and use the Additional Text button to include the report in your reply.

Then we can examine the report and see if we can determine what might be interfering with a more expedient download.

https://discussions.apple.com/thread/252829420?answerId=255322158022#255322158022

https://discussions.apple.com/thread/252825714

https://discussions.apple.com/thread/252802603

Now it's going to take over 4.5 days, no movement since last night. This is typical and I need MS Office and Adobe CC Suite for work.

No - only if it were M1 otherwise Safe Boot on Intel is SHIFT immediately at startup.

PS this method or that of @Old Toad should perform equally as well