Slow speed uploading using Mobile Client

Hello, I'm downloading and uploading files from a server (linux) to a phone (nexus 5) using the mobile client and using a router in a LAN with 50/50Mb (simetric - I know it's really simetric because I tested it with iperf). Everything goes perfectly when I download in the phone but when I upload I get slower throughtput (like 5Mb). I don't know why, and I'm even more interesting to know why than fixing it, because I've used the data for a project and I need to explain it... So, do you know what does this happen? Is there any known problem with owncloud phone client?