I dont understand Why the heck diaphragm changes only at the moment when the picture is taken its nonsense
And I don't understand why you would want to handicap yourself by attempting to create a composition using anything less than 100% of the available light that you can get.
What would stopping down the lens actually do for you? At moderate apertures in midday (f/5.6 and below), you'd get a preview of the Depth of Field, possibly at the expense of autofocus. At f/8 or smaller, you'll still get the preview, but definitely at the expense of autofocus and in addition to the image being so dim that you may be unable to effectively compose from scene to scene.
If you are worried about lag time for when you actually take a shot - the aperture stopping down is not a big contributor to this. The mirror movement is.
What does leaving it open get you? Plenty of light to compose and functional autofocus - which is necessary in today's digital cameras because we no longer get focusing aids (like a split/microprism). Even if we had those aids, they work much better and are easier to use with more light.
Attempting to compose with the aperture stopped down is a handicap for no real benefit. If you want to preview the DoF, then use that button to see and then move on.
Note that most seasoned shooters are using the DoF to aid fast shooting (f/8 and go street shooting) where compositions must be fast - too fast for previewing DoF. Or taking advantage of hyperfocal distance - plenty of time to check but you're better off taking the picture and then going to a 100% view to check for sharpness where you want it. Or going for some sort of maximum bokeh, leaving the lens open or just slightly stopped down - in which case the resulting image is not that much different than what you see through the lens while wide open.