I followed it exactly, except i am using a 1920x1080 background. Its wants to take 1330 hours to render a 4 second piece. after effects is also using hardly any cpu time. the station is a 2.93 i7 imac with 8gb ram.
Its does the same thing on the quad core mac pro.
its not settings because i just created, render and 1080 comp with particle world, it rendered, maxed cpu while rendering.
particle playground is not natively multi-threaded, nor can it use ae's 'render multiple frames simultaneously' option (as most other effects will), so any comp's that use that effect will render more slowly on multi-core machines... approximately at the speed of a single core.
there are a few other effects that can do this. you might check out some of these other tutorials here (and i'm sure you can find many more on the web):