Added memory does nothing to performance iMac, 27" screen.  Came with 4 gig of memory.  I never used all 4 gig until I started using IMovie.  Bought 8 gig extra memory, now have total of 12 gig.  Nothing speeded up at all.  When monitoring the memory, it seems that I'm always using over 8 gig and can only clean up the memory to about 6 gig.  How did I ever get by with 4 gig?  Nothing adds up.
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.