Conky is pretty old (though not as old as GKrellM), and someone else could probably give you a more technical description than I can. It's a (mostly) text based monitoring tool that is hugely (almost ...

update_interval: How many seconds Conky waits before refreshing itself. Even a "1.0" value shouldn't affect performance too much on a modern system, but if you're not looking for real-time system ...