I just finished reading this post: http://developer.yahoo.com/performance/rules.html#flush and have already implemented a flush after the top portion of my page loads (head, css, top banner/search/nav).

Is there any performance hit in flushing? Is there such a thing as doing it too often? What are the best practices?

If I am going to hit an external API for data, would it make sense to flush before hand so that the user isn't waiting on that data to come back, and can at least get some data before hand?

Thanks to everyone in advance.