Isn't it possible that some HDTVs apply some laggy processing to an RGB signal too, not just NTSC?
Hm...this is starting to go outside my technical comfort zone (read: feel free to correct or call me out on it) but yes. As far as I understand, just because the de-interlacing process is done, and even if the image sent to the display matches the TV's preffered resolution and doesn't need to be scaled up, it's still possible it won't go untouched.
So, yes, some kind of after processing might get applied. As you've said though, this is different between different units. As far as I've heard, even if one were to use the "game mode"-function, which is there to shut down/cut down on unnecessary image processing, they're not the same between brands or even different models within the same brand.
In short: even if you plug an RGB scart into the back of the display, the image will still need to be scaled. And even when feeding the display from an external scaler, it might still get processed by the TV, and this will differ from model & brand.