Yosemite and Parallels
If your machine is starved for RAM then it will be swapping pages of memory out to disk.

You can check this whenever you find your Mac stalling and performing poorly by opening Activity Monitor and checking the Memory tab. At the bottom of the window you should see a value labelled "Swap Used". If you regularly find a significant value against this then your Mac could likely use more RAM. Ideally you want this to say "0 bytes".

The coloured Memory Pressure graph can also tell you how efficiently your RAM is being used (i.e. is your Mac constantly having to swap things in or out of RAM). From Activity Monitor Help:

Green memory pressure: Your computer is efficiently using all of your RAM.

Yellow memory pressure: Your computer might need more RAM eventually.

Red memory pressure: Your computer needs more RAM.


Speaking from personal experience, Parallels (or any software that runs virtual machines) need a lot of RAM to perform well. Of course it depends on how much RAM you are allocating to your virtual machines.