Something might be quick at reading, but slow at writing and vice versa.
From the sounds of things, I'm going to say it's most probable that your PC HDD has slow write speeds or it's I/O is being used by another process while your NAS is trying to write to it.
Assuming you're using windows - go to Task Manager > Performance > Resource Monitor. Then look at 'Disk'. This should give yo a better indication of what's occurring during the write from the NAS to HDD.