Inventor Nastran (In-CAD) uses by default 2 processors (2 CPU cores, threads) for computing simulations. If you want to increase the number of used CPUs (cores), change its settings. Increasing the number of processors for parallel processing may accelerate computations of some models.
In the Nastran UI you can change this setting in the browser tree using the parameter: Parameters > Program Control Directives > NPROCESSORS.
If you want to change this parameter as a default, for any new analyses, you can modify the NASTRAN.INI file directly. You can find this file in the folder C:\Program Files\Autodesk\Nastran In-CAD 2018\System\Nastran\, resp.C:\Program Files\Autodesk\Nastran 2019\Nastran, resp. C:\Program Files\Autodesk\Inventor Nastran 2020\Nastran (depending on the Nastran version).
In the Nastran.ini file then use Notepad to change the NPROCESSORS parameter and save the change.