Dark Clouds can bring your game to a crawl unless you use optimised settings... for example you could have more random cloud formations... this decreases framerates dramatically... or you can have less random cloud formations and get about 75FPS or so...
I cannot comment regarding Tree Party yet, but going off the demo I can say it is pretty optimised and fast!
Remember a simple rule... reduce your polycount and use high detail textures and high FPS rates are always possible... just do not expect an old computer to handle too much so well...
Remember to have at least a dual core development system... preferebly quad core... and a graphics chip capable of at least Shader Level 3... 2 just does not cut it...
Hope that helps and be sure to get Dark Game Studio!!!