Thanks for your reply.
We have a scene with a very big relatively low poly landscape. On which a Forest Object with two Xref Trees is applied. The Forest has a limited altitude range and slope range to exclude the steep areas and the valleys.
The saving time was before switching to edit mode no problem. Then we switched to edit mode to render our animation. Before switching to edit mode for rendering we had problems with disappearing and reappearing trees.
Now we have to further edit the forest and the problem with the long saving times appeared. Also opening the file (with the forests on generate) takes a very long, no matter if the forests are visible or not.