the basic operation in openfx is to render a rectangular region of an effect at a given time. If every pixel has to be rendered at a different time, this would take an enormous time to render, for many reasons. The only viable option is to retime the source videos (not necessarily the input to slitscan) at a higher framerate.
My guess is that what you want to slitscan is actually a fixed image. then just increase the fps of your project and slow down the motion, you’ll get the desired effect.
The goal of these tricks is to cache more and compute less.