When my files get around 190 MB big the performance gets slow. Especially when using dynotopology. It starts lagging and sculpting won´t work in real time. Isn´t 8GB RAM enough? Is my Mac already to old? I can´t imagine how Games with graphics like Starcraft 2 can be displayed while blender gets slow at such comparably small files. Is there maybe a way to involve the NVIDIA GeForce GT (if it isn´t already) manually or something?
You can check to make sure the Nvidia GPU is enabled when you run Blender by looking at Activities Monitor. Graphics Card should say 'High Perf.' If you don't see the Graphics Card line (as the case with most non-laptops), your system is already using the fastest GPU available.