Full-Text Search slow

I have a CB mobile database with ca. 900 documents. One of the field of the documents contains the text to search for. The text in the documents is between 100 and 10.000 words. The property for the Full-Text search is indexed.

When I search for a 5 letter word that matches 5 documents it takes around 0.2 seconds which is not so bad. But it I search for a 5 letter word that matches almost all documents the search takes around 8 seconds which is way too long.
What is the problem here?