15 megabytes a second or 120mbits - means its probably connecting at gigabit end-to-end. If its only doing 100mbits then it would max out at about 11 megabytes a second. 

Therefore i am going to suggest its a speed issue with the NAS drive, source drive or individual files being transferred are small.    

In my expierence, linux samba file sharing has always been slower than windows-to-windows SMB sharing - though this is improving.  
And cheap single-bay NAS drives dont always have the fastest speeds.