I'd like to know more about your case, because my own experience is that ordering by ts_rank causes a big slowdown.
PostgreSQL documentation says: "Ranking can be expensive since it requires consulting the tsvector of each matching document, which can be I/O bound and therefore slow. Unfortunately, it is almost impossible to avoid since practical queries often result in large numbers of matches."
Some PostgreSQL developers are working on improving this by using indexes only to compute the ranking, but the related patches are not done yet.
What is the size of your data set (number of rows and size on disk) and the average response time?
