When you took the R9 380 4Gb card out of your system did you uninstall all of the ATI video drivers from windows.
If the answer is no, it is why you are getting poor performance from the Nvidia GTX 960 card you now have in the system.
If the case.
Click on the download link bellow, download the program, install it, then run it.
Remove all ATI video drivers first, and then the NVIDIA video drivers.
The program will remove both video drivers and any entries in the windows registry that will cause a conflict or a slow down in video card performance.
http://www.guru3d.com/files-details/display-driver-uninstaller-download.html
When both drivers for each type of card are removed.
Restart you system.
Go to the Nvidia web site, get the latest Nvidia driver for you video card and it`s model number.
Install the driver.
Then restart the system again.
Then test to see if the games you play run better Crazy.