[Bf-cycles] Cucles rendering speed difference between preview viewport and final rendering

[Le Jardin Digital]

Hi

can someone explain me this:

cycles (CUDA) render much much FASTER (x5 ) on preview viewport compared 
to the final rendering viewport ( with of course the same window size 
preview/rendering and the same level of samples for preview and render). 
What is the explantion beacause i don't notice quality differences...
It happens on all of my computers ( CUDA + win 64 + GTX470 + nvidia 
drivers 285.62), with every blender version (2.60, 2.61...)

Is it a way to make the final outpout as fast as the preview one ? it 
would be great !!

Thank you per advance for your responses.

 

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