hmm at least for me uploads are done to /mnt/dietpi_userdata/nextcloud_data/admin/uploads where they are stored until upload is complete. Afterwards they got assembled into the target file on target location.
Let me link myself as I’m to lazy to type thinks twice. This is basically similar to the other topic
https://dietpi.com/forum/t/nextcloud-slow-upload-and-freezing-when-uploading-big-files/6518/12