Well, then it seems clear to me that it’s not Nextcloud that’s causing the problem, but the satellite internet connection.
I also did a quick search and found this: Starlink Slow Speed: The Cause and How to Fix It, which shows that satellite internet connections generally have relatively slow upload speeds, in the range of 5-20 times slower compared to cable, and 20 to 2000 times slower than symetrical fibre optic connections, and on top of that there’s also a 2-15 times higher latency.