In summary, my project is a block based building / adventure game. The main difference between it and the obvious comparison (Minecraft) is that it's world has no ground. The only solid surfaces are floating islands of rock. The main goal is for a player to claim one of these islands and turn it into a mobile fortress with which to explore the world.

Right now I'm just working on world generation/display, and that's where I've encountered by first big problem. The picture I've attached is the basic idea for the look of the game world. I want the sky to be full of islands like these, but even this image is lagging horribly.

The generation and setup code is written in python, but I'm using Panda3D for all of the graphics (which is written in C++) I've tried a few tricks for increasing speed: billboarding the blocks when an island is a certain distance away, and billboarding groups of blocks as one surface when even further away, but these aren't doing enough for the lag problem.

My needs for an engine are simple, I just want to be able to show a bunch of textured cubes on screen. I would prefer python bindings, but anything will do. The only sticking point I see is that I want it to be cross platform. I develop on an Ubuntu (Linux) box, but the windows market is too large to ignore.

Any tips or suggestions are welcome.

I don't have an immediate answer, other than to mention that this very topic has received extensive discussion in some recent similar threads, so if you could locate those threads I'd imagine you'd find some useful tips. (Unfortunately I don't have any links handy, but searches for terms such as 'minecraft' and 'voxel grid' might turn them up. For what it's worth, I think one of the threads was in Game Programming sometime in the last month or so.)