Quite hard to answer without more details.

Are the clothes less resolution?
Les material zones?
Less textures at a lower resolution?
Les nodes?

An area render has to give the same result as the final result.
Area render or full render, all obj, all textures, all lights, etc, everything is always loaded to make the calculations.

IDL, IBL, AO, to get all the info, everything is always loaded.
Only final render time is reduced to the area.

In our minds, we expect it to be a "lot" faster but it is just a little faster.
 

Poser 1, 2, 3, 4, 5, 7, P8 and PPro2010, P9 and PP2012, P10 and PP2014 Game Dev
"Do not drive faster then your angel can fly"!