Lag in rpm drop

[knakki]

I have a 2001 2500hd with 6liter and a nv4500 trans.

 

Upon reving the engine it will not drop down to idle immediately, Either in neutral or in-between shifts.

 

This is annoying when trying to down shift and you blip the throttle to match gears and the rpms stay up high for a second then slowly drop instead of just blipping and dropping immediately.

It has done this since I've owned it back in 2006. No other vehicle I drive does this and it makes driving it awkward and uncomfortable.

 

Anyone know the cause or reason for this? Something I can do to make it drop fast? 

[Brendan Corr]

my automatic does the same thing if revved high enough, but it only hangs at like 2k for maybe two or three seconds.