How to speed up word count in tsearch2?

(It is the 2nd posting, maybe the 1st one didn't goes thru)
I've tested several keyword count from 2 millions record book
description table that indexed with tseach2 indexing.
The result is always slow for first query attempt.

This my sample query:
-- test one phrase --
SELECT count(*) from table1
WHEREsearchvector @@ to_tsquery('default' ,'david') limit 100
:: returns 16824 records match.
:: take 49618.341 ms (1st attempt)
:: take 504.229 ms (2nd attempt)

-- test two phrase --
SELECT count(*) from table1
WHERE searchvector @@ to_tsquery('default' ,'martha&stewart') limit 100
:: returns 155 records match.
:: take 686.669 ms (1st attempt)
:: take 40.282 ms (2nd attempt)

I use ordinary aggregate function count(*), Is there other way to count faster?