I am trying to do a simulation in blender in order to place some objects on a ground. During baking or simulation in real time everything is super slow (less then 1 fps) but CPU usage (6700K i7) is only ca. 30%. GPU 0%. I guess all simulations are calculated by CPU but why Bleder does not use it at 100%?
The simulation is likely only running on one core (with another core being partly used for scene updates).
Many (particularly older) physics simulators are single threaded, or at least some steps are. Multi threading in physics simulators is very hard due to all the possible inter-dependencies between objects, see also: Is Physics calculation multithreading possible?
Blender's rigid body engine is Bullet, which does support some multithreading, but I don't think that part has been integrated with Blender, and from what I hear, still has a fair bit of overhead.