I believe it does narrow the search results with each one via a map & reduce function. Johnathon Cervelli described this stuff to me in a SplunkLive a few years back and I believe it does a search where it finds the first term (map), then using that subset (reduce) it performs a search with the second term (map) and narrows the field down further (reduce), then repeats for each term. The one way to slow it down is to put it all in quotes where it then has to do a couple additional searches to ensure the terms are in order specified in your quotes.
...edit - oops, I meant for this to be a reply to your last message
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