While the performance of Unity running on current generation computers and handheld devices is still increasing, there are limitations to the amount of geometry you can place in an interactive scene. Especially trees, cars, curved models, and other objects with many polygons slow down Unity, sometimes dramatically. The following screenshot shows how to activate the Stats button on the Game view, which gives some insight in how much work Unity has to do.
We try to reach at least 30 frames per second (FPS) for smooth display. In practice, we hope to attain 60 or more and drop down to 30 as a worst-case scenario. However, don't be too fixated on this number, as it is not an exact prediction of how the application itself will run, for example, on a tablet or in the Web Player. The Unity Wiki provides a utility script for that purpose. For more information on FPS visit: http://wiki.unity3d.com/index.php?title=FramesPerSecond.
Luckily, Unity itself does some optimization...
