I can only guess.
When you are in full auto, the camera will use all 3 parts of the exposure triangle to end up with a decent amount of depth of field, a shutter speed that is ok to shoot in hand with the attached lens and will fill in with higher ISO.
When you switch to say aperture mode, it will let you chose the aperture manually and will control the shutter speed. BUT it may also allow you to set the ISO manually.
So if you are indoors and take said shot at
ISO auto, 1/25, f/4.0 and if ISO was at 25600, then the equivalent would be
ISO 100, 10 seconds, f4.0.
The only thing that is making no sense is that the cam has no such ISO capabilities. It only ranges from 100 to 3200.
So what I believe is, that the following happens:
You are set to a manual low ISO like 100, which triggers a slow shutter speed. This triggers noise reduction which then reprocesses the image and this results the long time that you perceive.
Quote from the manual:
• While noise reduction is activated, it takes about twice the usual
time to take a picture.
• The card access lamp blinks and [busy] is displayed
on the viewfinder while noise reduction is operating. You cannot take
more pictures until the card access lamp turns off.
If the perceived "lag" is always 10 seconds, then there is also an option to reduce shutter shock. This adds a delay after the mirror is flipped up to prevent shutter shock when using longer exposures. The option is called
anti-shock in the camera. Please also check, that this is off or at 0 seconds.
ANTI-SHOCK You can select the interval from the time the mirror is
raised until the shutter is released from 1 to 30 seconds. This
diminishes camera shake caused by vibrations when the mirror moves.
This feature can be useful in astrophotography and microscope
photography or other photographic situations where a very slow shutter
speed is used, and camera vibration needs to be kept to a minimum.
Notes • Anti-shock is added to the shooting functions individually
(single-frame shooting, sequential shooting, self-timer shooting, and
remote control shooting). “Sequential shooting” (P. 61)