Today 2021-feb-01 the download.nextcloud.com servers seems to bee slow (approx. 100 KB/s) IPs: 2a01:4f9:2a:3119::181 , 95.217.64.181
Same experience. My download / upgrade attempt failed. Stuck at step 4…
I went for manual Update by following this : Upgrade manually — Nextcloud latest Administration Manual latest documentation
But first make sure to delete the “.step” file in your [/var/data/nextcloud] subdir ./updater-XXXXXX
That file holds data about the last step taken by the web-updater
Manual download took a while, but worked eventually.
Now server performance is „back“ again - I could fetch the update package in less than 5 seconds… and the web based updater worked as expected.
Same here since yesterday. No update possible.
Today: [Exception]
Could not do request to updater server: Failed to connect to updates.nextcl
oud.com port 443: Connection timed out
Same issue here. Download speed from Index of /server/releases is between 50-60 Kb/s 
(tested from work network and from home)
It takes ~30 min to download 123 Mb
running at aws us-east1 (definitely no problem with the internet connection of the ec2. 
)
Unfortunately, this can happen directly after a release when a lot of people are downloading the image. I already did the upgrades, if someone needs the files for the NC21 release or the latest NC20 release:
http://dl.cl.cx/
In the data folder, you usually have a updater-data or updater-abcd folder, within that, you find a download folder where you can just place the downloaded file. The updater app should then be able to use this file. Or you do the manual upgrade: Upgrade manually — Nextcloud latest Administration Manual latest documentation
Please verify the checksums from the original download server (they are tiny to download)