This has been ongoing for me for the past month as well. Each Apple device I use (iPhone 11Pro Max, iPad Pro 12.9", 16" MBP, and 14" MBP) is bafflingly slow downloading updates from Apple—not from Adobe, not from MS, not from any vendor or client with whom I exchange large files—only software updates from Apple.
To illustrate, the 17.4.1 update on my iPad and 14.4.1 on 14" MBP have never been completed, no matter the time of day I try or how long I allow them to run. And this is across days of allowing them to try. All apps with software updates by MS and Adobe during that time have proceeded as expected, quickly, without undue pause or delay.
Re-configuring TCP and DNS and even rebooting all devices have not changed this. A few minutes ago, Adobe updated 3 apps in the CC in 10 minutes. Before that, Apple could not download a Pages update more than a MB or two after 15 minutes. After trying repeatedly over three days, the MBP stated 4 days for the 14.4.1 update. A Surface Pro 2 updated its OS and apps after 2 months offline in about an hour, using the same WiFi and settings.
Recap:
Multiple Apple devices cannot update Apple software despite hours/days of attempts.
Adobe and Microsoft updates of apps and OS all proceed without issue, on Apple and MS devices.
Apple and MS devices on the same network are experiencing vastly different outcomes, despite a stable network and 250+Mbps DL speed.
My next step will be to take a device or two to a wholly different location and attempt to download the recommended Apple updates, but the evidence thus far indicates a significant issue at Apple, not the ISP or devices.
EDIT: I should also note that updates to Parallels and the resident VM OSs and apps all proceeded as expected.