RENDER PREVIEW NODE (render half res)

I was trying to get a faster render preview using RENDER PREVIEW NODE set to half res ((trying to get the same result as Aftereffects displaying half, quarter, eight res previews))
However, when I put the render preview node in half res (or even eighth) after my final composite node, it takes as much time to render as the full res render…
does toonboom calculate fullres comp and then apply a resolution reduction?.. that doesn’t make any sense, does it? but It’s weird my pc takes the same time to render full res or eighth res
:’(
any ideas?
thankss