Thanks RegeHa & Sam. I am planning for music server with 2nd PI. I will try IP reservation in router. Even though my router Dlink DIR 300 and WiFi adapter are 300Mbps, receiving & transmitting @ 1 to 3Mbps only. I DO not understand where it is getting restricted. Somebody can help in this regard.
How are you testing and how are you copying/moving files?
Did you setup file sharing, Samba?
You need something that monitors network traffic or a simple tool like NetMeter on Windows.
Good ways to test are with WinSCP, setting up network drives and copying and moving from Windows Explorer.
I consistently get 60 mbps (7.5 MB/s) from Pi to PC and around 35-40 mbps between devices connected to the Pi i.e. pen drives, HDDs, etc.
For troubleshooting...
1. Connect the Pi via ethernet and test speeds.
2. Move Pi closer to router... distance will drop speeds when running on WiFi.
3. Use a WiFi analyzer (lots of Android apps) that will point out the best channel to use in your router.
4. The usual suspects like upgrading all the s/w and firmware.
PS: If you are planning to use the Pi as a NAS, don't. Its too slow, only 10/100 ethernet and USB 2.0. However, its excellent for media consumption... even 4K video will stream on it... and its excellent as a low power torrentbox, VPN server, etc. Most folks don't have ISPs of more than 100 mbps anyways.