Do you have the "Render Multiple Frames Simultaneously" option enabled in the prefs?
If so, AE is going through the process of launching the background processes of itself in order to make use of all indicated cores.
Yup, of course I'm using multi-processing. I'm saying that it's taking a looooong time to start the RAM Preview. Last week, it would start building the RAM Preview pretty much instantaneously. Now, it's taking 30 seconds to start building.
Yepp, as Steve said, depending on your memory settings, starting up the BG instances can take a while. However, when this pref was set when the program was launched, they should already be active and only require to load the project. Depending on how complex it is, perhaps it's more an issue with writing the temporary file and collecting the rendered frames from the BG instances. If it's a specific issue introduced by 10.5.7, it's still kinda weird and unexplained.
What Mylenium says is true: Once the background processes start up to take part in Render Multiple Frames Simultaneously multiprocssing, they should stay active until you quit After Effects...
> Yup, of course I'm using multi-processing.
There is one big caveat, though:
If you have turned on Render Mutliple Frames Simultaneously, but the background processes don't have enough RAM to render your frames, then they'll just shut back down again. And then they'll try to start the next time that you do a render (including render for previews). And then they'll just shut back down again. All of this wastes precious time.
The moral of the story: Don't think that checking the Render Multiple Frames Simultaneoulsy checkbox is the right thing to do in all cases. That's why it's an option, and that's why it defaults to being off. And make sure that you aren't starving the background processes of RAM.
Perhaps an icon/checkbox should be added to the Time Controls panel to add/disable multiprocessing.