That sounds much more in line with what I'd expect.
Here's an experiment: open Activity Monitor (should be in Utilities), and pin it to your dock. Go up to View -> Dock Icon -> Memory Usage. You'll get a pie chart in the dock, showing your system RAM. Red is active, Yellow is Inactive, Blue is Wired, and Green is Free. The first three are basically "RAM being used". Green is what you have available. Go about your workflow, but check on it when things start getting slow. Do you have any green? If you have available RAM, then upgrading probably won't help. With 50MB images, you theoretically probably have enough. As others have said, it depends what else you're doing simultaneously, etc. I've found that 12 is usually enough for me for LR processing, but when I run into trouble is when I have PS stitching together panoramas.
Sparknotes: If things get slow and you have no available RAM, an upgrade would definitely help. If you have available RAM when you experience slowdowns, the bottleneck is elsewhere.