didn't i understand sbillard right ?
sbilliard wrote: "...But that does not mean that the image is not being generated. ..."

Sbillard referred to the visiual thumb selector dropdown each album has. While Safari is not able to display the thumbs within as Firefox does it still generates the image.

i tried now to set the thumbnail to my default pic in each album without any pics and it works much faster. but when i set it to random-thumb it shows anyway the thumb of the only pic which is in this parent folder and not a pic which is in any child folder. i don't know the program-code but if it does what i think, whatfor it walks through the childs to generate a random thumb at the backend when it doesn't use it ?

Well, yes, it might be that we changed the random thumb to only use the image of the album itself and not child. I admit I am not familiar with the code in detail as sbillard coded that. He will be back in a few days and surely respond then.

Anyway, the main slowdown is if these thumb display needs to be generated and that for many albums wat once. The more image the bigger the chance it has not been already created (=cached). If then the images dimension wise are quite big and the server has problems to handle this adds up the more it needs to create.
Thus the standin image to avoid this on the backend.

The note that I made earlier was about the use of thumb images in the thumb selector. As said, it is not about if the thumbs show. That is a browser issue. If the option is set Zenphoto will generate them. Certainly if your browser does not show them that is a waste. Be sure that option is not set.

We have no plans to change the thumb substitute behavior. First, it is unlikely it is your problem. Second, it is a one-time action. If you just make the settings rather than complain you will have spent less effort.