It's true that JavaScript code still isn't as fast as what you can make with C++ (perhaps by an order of magnitude or so), but I've recently seen demos of a Quake III map renderer, a Quake II port and a 3D physics simulation that convinced me JavaScript was probably fast enough to make serious games. Generating simple worlds, meshes and textures procedurally would free me from one of the main issues I've had in my past game development attempts. The algorithm I implemented unfortunately scales fairly poorly, however, as it tests each possible combination of 3 bounding planes for intersection to find the vertices of a given brush, which has expected O(n^3) running time.