Also, keep in mind that Arch is a rolling distro (just as another favourite of mine: openSUSE Tumbleweed. I run both on different laptops), so the situation is hard to compare.
Rolling distros release tons of very small update very regularily. At worse, if you’ve forgotten your weekly update, the week after that you might be looking at ~100 packages to upgrade (real-life example based on the logs of the Manjaro ARM derivated of Arch running on my PineBook Pro).
Sailfish OS uses full-blown complete releases, and is in a phase where there are trying to catch-up the versions of most software, including core stuff like the gcc compiler and libc. Which means that the whole 1.5 GB of your root patition get completely replaced on each major upgrade.
That’s a quite significant amount of packages and data that need to be upgraded.
Also, regarding network, keep in mind that you’re not alone, all the other SFOS users are hitting the CDN similarly hard, everyone of them trying to pull this 1.5 GB worth of updates at the same time, because you all got the same giant update (Whereas, in my example above, probably the other Aarch64 users got some of their updates last week already and by now probably only had a couple of dozens of new package to upgrade).
Note that this also happens every now and then on rolling distro: a few core elements changed last week in Tumbleweed, meaning that I had >6000 RPMs to download and update on my work Dell laptop. Whith every body need to update significant portions of their system the same week, the download servers were similarly crawling and the connection was dropping regularily.
So yeah, some efforts could be done on the CDN side to try to aleviate the situation, but part of this is also due to Sailfish OS not being a rolling distro.
Though once Jolla’s devs they finish their catchup and are running more recent versions of everything, one could expect less dramatic change between version (only a subset of all the packages needing to be replaced each two-month), that still won’t be the “couple of dozens package per week” that we’re used with our rolling distros, but certainly Jolla’s roughly bi-monthly upgrade cycle is going to be less massive compared to Ubuntu’s semi-annual releases.