[pacman-dev] Variable execution time for pacman -Qdt?

I have noticed that the first time I run pacman -Qdt after not running it for a while it can take a variable amount of time -- usually at least 20 seconds and as long as one minute. If I run it again shortly after that it completes in less than a second. Is there a simple explanation for this variation? I have 1705 packages installed and pacman -Qdt lists 26. The reason I'd like it to be fast is that yaourt calls this command before a system update, so I get a 20-60 second lag before being prompted for my password for sudo pacman -Syu.