Canon EOS 5D Mark III Instruction Manual (en)

 Noise Reduction SettingsN
With [Auto] and [Enable], after the picture is taken, the noise reduction 
process may take the same amount of time as the exposure. During 
noise reduction, shooting is still possible as long as the maximum burst 
indicator in the viewfinder shows “1” or higher.
Images taken at ISO 1600 or higher may look grainier with the [Enable
setting than with the [Disable] and [Auto] settings.
With [Enable], if a long exposure is shot with the Live View image 
displayed, “BUSY” will be displayed during the noise reduction process. 
The Live View display will not appear until the noise reduction is 
completed. (You cannot take another picture.)