Canon EOS-1DX Instruction Manual page 155

3 Noise Reduction Settings
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] or [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.)
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