Added memory does nothing to performance
The (good) old days where you had to manage your RAM usage are history.

Apple OS X does that for you.

Keeping free RAM is thing of the past. It did not make sense anyway to keep unused RAM.

The OS X manages the RAM for maximum efficiency, and speed.

In doing so it will load the RAM up to what is needed. 

If your system appears to be slow, it is usually not the RAM, but some process eating up the CPU.

You can look up in the Activity monitor to see what is it.