I was having this problem because my sims' house and inventory was full of photos they took and paintings plus all the "cd" cases from the uploaded music they created. Apparently, I learned recently that your sims being overloaded with stuff they made is one of the causes of them taking a long time to do things, staring into space etc. I now have a sim named Arthur Curator who lives in an art museum of sorts where I store my sims' self-portraits, photos, etc. This has all be completely fixed that problem.
