In Beowulf, the author is trying to present the listeners with the idea that this monster is something different from all the other monsters they have ever met, be they wolves, snakes or sharks. This monster is "other-worldy" and has something of the supernatural about it. This is because the cleverer and faster the monster then the cleverer and faster the "hero" who manages to vanquish him - in this case Beowulf. The worse the monster he overcomes the greater hero he is in the view of the listeners to the tale.
So, not only is this monster ruthlessly blood-thirsty he is almost impossible to outwit because he has brains as well as brawn. With lightning speed his mind is calculating his next move in the struggle just as Beowulf grapples with the last.As fast as his greedy claws are tearing, his next plan is racing. Beowulf must "face the monster" by "fighting fire with fire" as our modern saying goes.
The line you quote is from very early in the epic. Hrothgar has built Herot and the noises of the building and the parties have annoyed Grendel.
So Grendel comes and, in the part you quote, he grabs up thirty men and kills them. He then carries them off to his lair.
The line you quote is meant to show how fearsome and evil Grendel is. Look at all the things that are "quick":
- His greed -- this means that he is insatiable, willing to kill so many men (to eat?)
- His claws -- this shows us that he is ferocious and terrible, with quick claws that can kill in an instant
- His thoughts -- he doesn't take a lot of time worrying or deciding. He sees what's going on and springs into action.
So when your adversary is ferocious and greedy and really quick-thinking, you are in a lot of trouble. And that's what the author is trying to get across with this line -- Grendel is trouble.