This may have been suggested already. But it would be nice if when in the filter editor you could draw a marquee box around a area in the preview and filter forge would only then update that area or update that area first to give faster preview results of the area needed. Maya has this feature when rendering.
A quick workaround -- you can zoom in on the area you want to be rendered first. Filter Forge renderer places higher priority to rendering blocks that are in the view rectangle.