Big Sur 11.4 downloaded extremely slow (20 hrs), then failed via "Software Update". Workaround was to download from Apple store

All update routes lead to the same Apple server. Yes, corruption can sometimes occur. When it does, it is due to an internet issue or an interruption in the download process. That is why Wi-Fi should not be used if possible. it is too slow for these very large files. Also, it can be possible for a download to take longer simply because the Apple server is overwhelmed at that particular time. Finally, you are not communicating with Apple here. We are Apple users like you are. If you have comments for Apple, connect to them at the following link: www.apple.com/feedback

Sat here staring at 1.29Gb of 12.38Gb - About 2 days remaining -

I have a fully working 100Mb DOCSIS line, 1Gb Ethernet Router connected, I can download Windows 10 in minutes....

Come on Apple, it ran OK up to around 900MB then literally ran into a brick wall

Thanks for this took me a couple of days of pain before seeing your post, but 30 mins later back up and working. Apple something is wrong with the update through the proper route ie ‘Software Update’

Same for me. Update failed when using Software Update in System Preferences, but worked when initiating via the App Store. Thanks for suggesting that route, sirsycamore.