VERY slow searches when many urls are returned

I have this huge problem with search speed.
Whenever a query gives many results, the search takes too long.
For example, search for 'work' gives like 30k of links and takes 18.5
seconds. And search for 'good work' gives about 12k docs and takes less
than 2 secs. Searches that give few 100's of links take about 0.1 of a
second!
What is the problem? All CPU cycles are consumed by searchd during the
search time, so it is not the database. I encounter exactly the same
problem with aspseek search.
Please, help.


Best
Max