Using Ramcache III app but I believe there are other apps that do the same thing, but with ramcache you just select the storage device and move the slider along to say how much can be used for caching data. It basically makes it quicker for the CPU to read the data than getting it from the storage device.
As I have 64gb I have set the cache at 32gb sim will use upto 20 gbs then leaves a little spare. so ram usage goes upto around 52 gbs when using the sim.