[Gavino]

Originally Posted by Seedmanc

...

original=last 

last.BlankClip(width=last.framecount-1+last.width, color=$888888)

scriptclip("""
        for (i = original.framecount-1, current_frame, -1) {
            frozen = original.freezeframe(0, original.framecount - 1, i)   
            layer(frozen , x = i,level=128)               
        } 
 """)
crop(300,0,0,0)

...
If only there was a way to massively accelerate this processing, it would have became a very useful tool.

A variant of the trick from the 2009 thread "Sticking" overlay might help.
The idea is to reuse each partial stack in the construction of the next rather than building each one from scratch.
For this to work, you will have to generate the clip frames in reverse order, so run the following script, save the result and either play it backwards (if possible) or run another reverse script to generate the final result.

For the script, replace your scriptclip section with the following code: (warning - untested!)

original = original.Reverse()
stack = last # the blank clip
ScriptClip("""
  frozen = original.freezeframe(0, original.framecount - 1, current_frame)   
  stack = layer(stack.SelectEvery(-1), frozen, x=current_frame, level=128)               
  return stack
""")

The performance gain comes from the expectation that at each frame, the previously generated frame (obtained via SelectEvery(-1)) will be preserved in the Avisynth cache.

The frames have to be generated in strict linear order, so it probably needs to be run single-threaded.