What's wrong with MotionBlur?

I tried to use blur for motions of my character, exactly when he jump, but build-in motion blur, are so slow. I have 1GB of RAM, and GF6600 (256 PCI-E), and one core Celeron processor (3GHz). So I CANT use build motion blur with this computer? Is there any alternative for archive motion blur?

blurring individual objects via shaders.

That's about the only viable way. Construct's built in motion blur just draws many frames and composites them together, so for 5x motion blur you get 1/5 framerate.