I have also followed Spaceinvader One's guide and having sort of the same issue and i can't understand why. Copy within the docker console works as expected, 5gb file copies in 8-10 seconds.
I am getting slow uploads only (10MB/s), downloads are fine (90-120MB/s). There are som topics concering this issue over at Nextclouds forum but I am not getting anywhere with the solutions suggested over there.
https://help.nextcloud.com/t/slow-upload-speed-i-need-advice/60909/14
I can't remember this being an issue looking back on using Nextcloud for almost 2 years. I had some trouble uploading large files and had to make some edits to the .conf file for swag to fix that.
Other from that, nothing has changed.