Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 192)

3 Setting Noise Reduction
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With [ Auto ] and [ Enable ], the noise reduction process after the picture is
taken may take the same amount of time as that for 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 high ISO speeds may look grainier with the [ Enable ]
setting than with the [ Disable ] or [ Auto ] setting.
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.)