It’s that blurring step that’s going to kill you. If you’re not using a shader, that means you have to blur the texture on the CPU, which means you have to copy the texture RAM from graphics memory to main memory and back again, and that double-copy is death to your frame rate. Plus the CPU isn’t as good at that kind of processing as the GPU is, so you get a double hit.
It would probably be better to use an on-card approach to soft shadows, rendering multiple different (slightly offset) views from the light onto the same texture, blending them sensibly. But this means multiple rendering passes over the scene, which can again be a killer if your scene is complex.
David