Each time I change the lens I like to turn camera off so that protective curtain close and prevent dust from coming into contact with the sensor!

However, it takes around 5 seconds for the sensor curtains to close.

Why would it take so long? Shouldn't curtain come up as soon as the power is turned off?

I wonder if this is unique to my camera, or to this model of the camera?

Any experience or views would be welcome.