Upload of tiny file is insanely slow. How?

When I upload a txt file to my nextcloud server via WebDAV that only contains a single “1” (thus, 1byte/4kb filesize), it takes about 40 seconds. Why is it so slow?

I mean I only use a Raspberry 3b but… can it be THAT slow? Why? I mean if I upload a larger test file (~134MB), it also takes about half a minute to start, but when it’s going, the activity monitor shows a continuous network usage of 3 to 7 MiB/s. Not quick but ok.

Everything is as first installed, only 1 user and only the files that show up for every new user.

Using nc 15.0.4

Would appreciate every hint and/or idea.