Canon EOS R3 Advanced User's Manual page 254

Caution
With [Auto] or [Enable] set, noise reduction after you shoot may take as long as
exposure for the shot.
Images may look grainier with the [Enable] setting than with the [Disable] or [Auto]
setting.
[BUSY] is displayed as noise is reduced, and the shooting screen is not displayed
until processing is finished, when you can shoot again.
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