You mean writing to the NAS is slower than the local drive? That is expected, but it is hard to say if it is within normal parameters or not without some hard numbers.
What is the MBps read/write time are you getting to the mounted drive?
You can get a rough idea by mounting the CIFS share, and running the following command in it:
dd if=/dev/zero of=./test bs=1M count=100
This will create a 100 MB file from your local drive to the CIFS share, and give you a report about time taken and write speed.