Mac performance in object or edit mode


I have an iMac, 4GHz intel i7, 16 GB memory and I have a problem.

When I do stuff (duplicate, remove, move, separate, undo) in object or edit mode, Blender can be really slow, like ‘not responding’ slow. I found out that Blender uses only one core out of four. This happens also when rendering a scene. Only time when it works with more than one processor core is, when I use rendered preview in 3D view.

Thank you and have a nice day