Hello Roberto ,
Here is the explanation I received from the developers, that not all of SpaceClaim is multi-threaded, and so it doesn't always use all the power available on multi-core PCs. Loading .scdoc files is mostly single or dual-threaded, so it will use the equivalent of one or two cores, which will be around 20% CPU usage on a 6 / 12 core machine like the Intel I7-10750H on this user's machine.
Regards.