Dear Astronomers: […] I've poured through my roughly 200 astronomy books and can't find an answer to this question asked by a boy at one of my recent talks: Why do the outer gas giant planets [generally] rotate faster than the inner planets? I was shocked at first because I realized I should know that! The best I could come up with was that the planet forming gas/dust around the bigger planets just happened to have faster moving eddies of material there than in the inner areas of the forming solar system. But I'm not sure that's it. Any ideas?
What a great question! Here's my thoughts on the subject, I hope they help: So the rotation rates of the terrestrial planets, ice-giants and gas-giant cores are largely determined by the stochastic collisions near the end of the formation process (when the colliding bodies are largest and so impart the most angular momentum). This effect is largely random and so will give a certain average angular momentum but has not inherent preferred direction and so won't lead to huge rotation rates.
Meanwhile the rotation of the gas giants is determined by a combination of the net angular momentum of infalling material during it's runaway growth as well as asymmetric inflow of material during the final stages of growth when the gas giant has largely opened up a gap in the gas disk it's forming in (the same process that generates type-2 migration). Looking at this first effect, the fact that gas giant planets are formed by so much material/mass which starts out relatively far apart will give a fairly large angular momentum depending on the eddy speed as you pointed out. On top of that, the fact that the late stage inflow onto the gas giants is in a single sense (clockwise OR counter-clockwise from the inner AND outer inflows) will spin up the planet's rotation rate significantly.
I hope this helps answer the question to your satisfaction.
University of California, Santa Cruz.