Hi Tim,
sorry it took so long - been busy.
By “not magic” I mean that you will by definition lose detail if you downscale. There is no trick to avoid that. Sorry if that ruined your day! 
The preview will probably also not help you since you said that downsampling 3 is already to bad for you, preview is 4 by standard.
The time is a lot shorter. Impossible to put a number on it but I’d say between a quarter and 10%…