I was surprised to find beef-witted as a suggested translation of schwerfällig. I've never seen the English term and searching online dictionaries (OALD, CALD, Meriam-Webster's, AHD) gave no hits.
Further searching shows that it might have been listed in the 1913 Webster's http://dictionary.babylon.com/beef-witted
Answers.com suggests it's from Shakespeare: http://www.answers.com/topic/beef-witted
From Troilus and Cressida, Act II, Scene I, apparently. "Ther. The plague of Greece upon thee, thou mongrel beef-witted
While it certainly seems like a useful insult (and where better to find them than Shakespeare), I think it could best be described as dated, and perhaps marking it as so would be helpful. Otherwise someone searching for a completely normal translation of schwerfällig might think beef-witted is the best choice.