Hmmm, GTX 550 Ti should be fairly powerful for such a simple scene.
?You should implement a triangle count. From my experience, there is nothing you can do to a make an under 1 million triangle scene that uses a lot of vertex buffers run like crap and if it does, there is something wrong somewhere else. Either on the CPU side, or you are doing very heavy texture sampling. Or cascaded shadow mapping.
But for environmental mapping, you can use a fairly low resolution and very blurred image. You can omit small objects, use cheap filters, omit normal mapping, render meshes with a lower LOD, etc.. You could also recreate the cube map only once in a while, or when some object close to the environmental mapped object changes. You could render one face of a cube a frame, so you have a new cube every 6 frames, reducing you draw calls to 2x. Also, you should have very good batching behavior, but if you only have a few objects, this is probably not your bottleneck.