Well, you might want to read the many, many "How do I get maximum performance?" and similar threads that have amassed here and elsewhere. The long and short answer is that there are limitations due to how AE works. But really, it's too tiresome to go over this again and again, so a Google search for the respective keywords is the best you can do to spend your time and spare everyone another nerve-wrecking, longwinded post.
Thank you Mylenium,
I would rather spend my time working on my project instead.
And I already did what you suggested for some weeks now, without succes.
Background render does not work on my machine.I know of multi core effects.
I hope some one with specific knowlegde will respond to my post.
The simple fact is that After Effects will not max out your system resources for most projects. Only a few combinations of effects, codecs, and plug-ins will come close. Adobe has a long way to go and processing video one frame at a time the way AE does cannot use all system resources all the time. Think about it. A single frame of HD video is way smaller than the available memory on any system built in the last 10 years and the time it takes a CPU to process that frame before retrieving the instructions for what to do to the next frame is very short.
I would look for improvements in future generations of AE, but for now, in general, unless you know that you are not using temporal effects or other plug-ins or codecs that work well with MP rendering, turn it off. IOW, if you have any lagging or locking up with MP turned on, turn it off and don't waste time messing with it. Keep your memory allocation in multiples of 4GB if possible, make sure that the actual processors used and the number of processors in your system add up. I've seen so many folks try to allocate a bunch of ram to processors in MP rendering and end up with 0 processors used. Just a little common sense and about 2 RAM preview tests is all it takes to get your system running as good as it is going to get.