Nextcloud 15 hangs when there are too many files in a folder (pictures in my case)

Hi!! i am new in nextcloud and i have recently installed on my Raspberry pi 3. First of all, it run very fast and very fine, but now, i have got a folder where i upload all my pictures from the phone. It has about 50Gb of pictures and videos and the nextcloud hangs when i try to upload there some photos. I can upload 1 picture and then, all the Raspberry pi hangs for a time (i really do not know the exact time it hangs, but it can be about 20 mins or more). This problem is present when uploading 1 file to that folder. If i upload a file to another folder with less files, the problem does not occur.
The Gallery app hangs the Raspberry pi too, and no thumbnails are shown.
I can reproduce this problem using a web browser but I think it not happens using the Android nextcloud app, or i could not reproduce it yet.
Does anyone know what am i doing bad?