Slow loading data from client

good evening guys,
on my old PC (celeron 2GB Ram) I installed ubuntu 16.04 and owncloud, everything went perfectly, I can upload and download data very well.
But when I try to do the same from the PC to use as clients (MacBook Pro Retina) often loading freezes, I can not upload more than 10 files at a time or else freezes, although I set the upper limit to 2gb...
How can I do?

Without logfiles it is hard to tell what is causing this issue (could be a real error or not optimized configuration):

On the client, you can press F12 to open a logging window for more information.