[Brightcolours]

Longer focal length does not introduce lag...

 
No, it just exaggerates it!
In the same way that longer focal lengths require faster shutter speeds I suppose!! ....... I wonder why that is..... it couldn't have anything to do with magnification could it? ... no, of course not ..... it's all the same! ...... and stabilization is the same for all focal lengths ... fish eye to 1,000 mm ...... those manual FL settings for lenses that don't communicate to the camera when using IBIS are just there for show ... when we all know that one size fits all ......
I will be looking elsewhere for long lens shooting information thanks all the same ....

1. Mixed bag Tony and mixed bag female Tony showed in that video the "issue" which was entirely their fault: they saw the taken image due to an image review setting, making it impossible to follow a bird (or anything moving) after taking a photo. Their criticism (as so often with certain brand "product assessments") was rather nonsensical, as mr Polin's video and the other video I linked to shows.
   
2. If the live view lags, it does so in the beginning too. So, the bird will never be where you see it and whether you try to take the 1st frame or the 13th frame, the result will be the same. The lag would not multiply after every image.
   
3. Yes, the mechanical shutter black out is visible (as seen in mr Polin's video). It is not longer than you get with your D500 or a D850, but with the DSLR you get less on/off effect when the mirror moves up and down, you get a more gradual and at the same time more flickering effect. With the electronic shutter you get NO black out or lag what so ever when taking images. 
   
4. With a longer focal length, a small movement of the camera/lens makes for a bigger movement in the frame. That is the reason why exposure times need to be shorter and IS is more helpful at longer focal lengths. Lag, however, remains exactly the same. Whether you photograph a car passing by with 50mm, or a bird flying by with 500mm, 
   
5. Cameras with IBIS can use any lens with the IS, you just need to tell the camera which focal length you are using.
   

If you still wonder why I said Tony is a mixed bag himself...
I wonder which incorrect objections and "facts" you will post next....