I was wondering if adding more RAM would help the stalls and poor performance of a mid-2011 iMac. It was working great until I upgraded to Yosemite and the newer 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.
Speaking generally, adding more RAM gives the operating systems more resources for managing applications in an efficient manner. However, with respect to using Parallels, I'll add the following:
I've used Parallels for a few years now, with both a mid-2012 macbook pro (8 GB RAM), and a 2014 macbook pro (16 GB RAM). As a rule of thumb, you want to share the following evenly between Windows and Mac: CPUs, Memory, and Video Memory.
By equal sharing of resources, neither operating system should be starved of resources, especially when you're using many applications on both systems at the same time.