I'd take a look at the MTU size (sometimes called Jumbo Frames) on both the NAS and the Windows client making the copy along with any network devices between the two systems. This frame size can improve network performance between two system dramatically. Without knowing the exact NIC card driver and NAS hardware it makes it hard to give specific on how to change it.
Changing the MTU size is complicated because there is no standard on sizes for drivers. Drivers in FreeBSD, Linux and Windows are not consistent with what sizes are allowed. You may have to dig into the driver documentation or experiment to get the sizes that are acceptable. Also, lowest value is the default for the entire path between two systems. The network devices between them (router/switch) will have to have jumbo frames enabled as well.
On your windows machine try right-clicking the NIC in device manager and look in the properties of the NIC driver. MTU or Jumbo Frame or Framesize may be the name of the setting for your driver. The default value is usually 1500.
On your NAS device, you will need to look around for a MTU, Framesize, or Jumbo frame entry and enable it. This is usually in the advanced section of the LAN network configuration.
Some notes will mentioned hardwiring full-duplex for your NIC but most modern switch will deal with this quickly and not be a problem. I have not seen any issues with duplexing on modern hardware post-2000.
Please let us know what NAS device you are using and what version of Windows if the above doesn't help.
I hope this helps out.