Why is my network transfer being throttled down

My laptop and OS is:

OS X Yosemite

MacBook Pro (13-inch, Mid 2009)

2.26 GHz Intel Core 2 Duo

4 G, NVIDIA GeForce 9400M

I have Airport Extreme router, the flat square kind with USB-2 port.

Here is my problem that occurs across several configurations:

I am trying to copy my iPhoto library to a network drive; the drive is hosted by the Airport router. The file is about 40 GB. I expect it to take some time to copy. I open up to Finder windows, copy and past from my hard drive to the network drive. The copy starts off great, it moves about 10 MB per second (sometimes a little faster) . So it should take about an hour and I'm fine with that. However, the amount copied over hits the magic number of 220 MB, it stops transferring 10 MB per second and throttles down to .1 MB every couple of seconds. It is very painful to watch. Then it updates the time to copy to about "a day"; that I'm not OK with.

Configurations that show this behavior were:

NAS plugged into the Giga bit ethernet port of the router

NAS plugged into the Giga bit ethernet port of my laptop

USB drive plugged into USB port of the router.

Please help