Shutter drag problems

Check to see if somehow the "noise reduction" setting is on. This in effect takes two separate exposures on each shutter release, so it (sometimes) doubles the time the mirror is up. I'm not sure where it is in the menus on a D300.

Also, just a terminology point, but the "shutter" in this camera is a combination of the mirror (what we usually call the shutter) and the electronics which samples the sensor for a limited time. It's a little like the mechanical shutter on a film camera that was behind the mirror....back in the day when they made them from titanium.

The "so what" of this is that even if it sounds like it's not releasing as fast as it used to, the exposure can still be accurate if the electronics are sampling correctly. It's only when moving to slow exposures that the mirror speed/up time makes a difference.