uhhh JBOD doesn't do parity, unless you're running something really weird with it.
Anyways:
assuming a 24gb 90 minute movie:
That's 267 MB/Minute you're pulling off the NAS.
1gbit networking can move 6,000 MB/Minute. So, no, you're not going to saturate your ethernet with movie watching.
If you're just playing the video on a PC, and not transcoding on the NAS, there should be very little CPU Load from streaming the file to your PC/MAC.
I'd check to see if all your drives in the NAS are healthy first.