thank you for your support!
I tried to deactivate the Category Listbox, but no changes for this issue.
The same with the level of categories.
The load times for download details pages is normal.
It's very slow, when loading a category or any page with a module (f.e. Featured or latest Downloads).
Before Joomla with JDownloads Component i had an older Software for my website with the same content.
The load times were <1 sec., i think about 6-7 Second is much too long.
It will be great if you could have a look at the DB-query, creating the sort index takes much of the time.
It seems that the whole DB is sorted, but it's not needed in most cases.