[kmike]

Originally Posted by DigitalCrowd

Well, then you get into having to fetch the current lastpost field of all matching threads and that would be, on larger boards, significant overhead. Now you move away from just searching Sphinx, to now search the database as well, then sorting your resulting arrays and pretty soon... you have a SLOW search again.
....
Without adding overhead to the search process, I think the days of instant lastpost sorting are gone, unless you can rebuild your full index every 15 minutes or so and for some people, that index process might last that long or longer.

Actually there is a function in vB which does exactly that (sorts the results by the specified field):
sort_search_items() in includes/functions_search.php
It's just one query, and it runs on a slave server if it's set up. Also its overhead depends only on the number of returned search results.
And you don't even need to run through it on every search, only when listing the results as threads, sorted by lastpost.