As a Followup to this (Bad performance with grouping)
Forumpost I managed to reproduce the issue now.
Since i cannot upload zip-files in the Forum I am doing it here.
The problem is, that a single grouping dramatically increases the time for sorting the values.
When the grouping is not present, sorting the same column is much faster.
It appears that it is related to the ammount of columns in total (not grouped).
The ammount of records is not so important, 100 (as in the attached sample) should not be significant.