This has been bugging me for months… whenever I opened up the Terminal App on my iMac it took a good 30 seconds to do anything. I only just now, whilst waiting for it to load, bothered to Google the answer. Apparently it makes use the asl directory underneath /private/var/log. ASL is an acronym for Apple System Log and relates to one of a whole raft of system related logging. These logs can safely be cleared although it might be worth taking a backup just in case.
Running the following code will, in effect, delete the contents of that directory and all things using that directory in a reading capacity (working with the logs as opposed to writing their own) should speed up.
sudo rm -f /private/var/log/asl/*.asl
Note that this is non recoverable, hence my suggestion you take a backup first. Hope it helps someone else like it did me!