Why App Store & macOS updates downloads are so slow in macOS Sonoma?

I have a 1000/300 Mbps fibre connection. On my 2023-Nov 16" M3 Max MacBook Pro (which has Wi-Fi 6E), I have been experiencing slow download speeds exclusively from the Mac App Store and when downloading macOS updates. Other download events' performance varies, but it is always more than acceptable. For example, using the Steinberg Downloading Assistant, the speed can reach 100 MB/s (close to the theoretical maximum of my network). When I download an app, or an update, from the Mac App Store, the maximum is capped at 2.7 MB/s. If I press the Update All button (which allows up to 3 concurrent downloads), speeds go down to 800 kB/s... While this clearly isn't a hardware problem, I ran Wireless Diagnostics and it showed no issue, as expected. I have another Mac, running Monterey 12.7, connected to the same network. It has Wi-Fi 802.11ac (compared to this new 802.11ax). Holding Option and tapping on the Wi-Fi icon in the menu bar gets a Tx Rate of 867 Mbps (compared to the new one which has 1200 Mbps). Downloading macOS updates or App Store updates/app there is blazing fast, 50+ MB/s. I am using iCloud Private Relay on both Macs & am using CloudFlare (1.1.1.1) DNS as top priority on both Macs. Another Mac mini in the house (2018) is also downloading well. So, it's not the network, it's not the Wi-Fi modem in the new Mac, what is it, then? Is Sonoma to blame here? Is macOS using a specific port for these two services (softwareupdate & the one governing Mac App Store updates, assuming it is different)? Apple service is not to blame, since it works in an older (Intel) Mac and not in a newer (Apple Silicon) Mac. Thank you for your insight. EDIT 1: following Apple Support initial contact, I created a new Administrator account and tried again there. Same results. I then booted into Safe Mode from the main account and tried again here. Same results. Did a Speedtest again and, generally, I'm getting 700+/300 Mbps speeds. EDIT 2: second round with Apple Support today. We tested a tethering connection with my iPhone XS. Speedtest gave 30/10 Mbps, so about 25x slower than my home Wi-Fi. Using the App Store, I began a download event and the speed was 4.2 MB/s, perfectly in line with the speed test, and almost double the one I had with my Wi-Fi. Now, Apple Support suggested I contact my ISP to see if they have some protocol that may prevent just the App Store & softwareupdate from using the whole band. But how should I word this? I need some technical language I am not proficient with, and also, why this Mac has this issue and the others, older ones, do not? Could this Mac's Wi-Fi 6E modem not like my 5G router, but only for these processes? If so, what could they do to solve this? Are there specific ports being used by App Store & softwareupdate? If this brings no result, they suggest reinstalling macOS.