help me with opengl accumulation buffer

(sorry for my must-be awkward English)I recently used opengl accumulation buffer to implement motion blur effect in my real time application in which the detail is very simple. Only for checking my supposition, I use the combination of the only two successive images to make a motion-blurred one. The first image is stored in accumulation buffer, and after the second one being accumulated into the buffer and displayed, the buffer is replaced by the second image. This cycle continues. But the problem is that the operation on accumulation buffer seems to be very time-consuming and the fps is curiously low. Is this caused by the low-efficiency of operation on accumulation buffer given that the scene is very simple? But I believe that accumulation buffer should be very high efficient. Why? Please help me.

while(1) {
glAccum(GL_ACCUM,0.5);//combine the 2
glAccum(GL_RETURN,1.0);//display it
glAccum(GL_LOAD,0.5);//this is not
//efficient, but it is for

[This message has been edited by hapcafe (edited 03-14-2002).]

Accumlation buffer is always very slow. Sadly, there is no motion blur effect out there on the market that is very fast these days.