optimizing count statement

[Karolis]

hello,

I am working on a search engine PHP script and I have a problem with select count(*) performance:
The table I am working on has 2 columns:
KeywordID (int), DocumentID(int);

I have created indexes for both of those columns.

My SQL statement is as follows:
SELECT COUNT(*) AS Cnt FROM TABLE T1, TABLE T2 WHERE (T1.KeywordID = 7 AND T2.KeywordID = 79 ) AND (T1.DocumentID = T2.DocumentID );

Problem:
when MySQL finds no more than ~3000 results, the query is executed in ~0.01 seconds with about 1 million items in the table. However, when there is more than 4000 results found, the query takes about 2 seconds to execute.

Is there any way to optimize this selecting this count?

thanks,
Karolis