When did you start using it? :-)
I do that for long and most of my time consuming queries are marked by SLOW_OK. This one started being killed just recently, that's why I noticed this just today.
It could be that this query did not work too fast in the past but at the same time it did not happen to it to be killed, so I conclude it became slower recently.
Does my SET SESSION TRANSACTION ISOLATION LEVEL READ UNCOMMITTED; improve usual MySQL speed on count(*)?
mashiah