Optimization Utility uses Autodesk® Nastran® 2018 in an integrated form (without a user interface) to solve the simulation and optimization tasks. You can learn more about the Nastran solver in the Nastran online help.
There may be cases where modifying the internal Nastran settings can be beneficial to improve the performance. Specifically, simulation jobs with a large number of degrees of freedom (tets, forces, etc.) and calculation times in the range of hours at default settings could benefit from adjusting the number of CPU cores available to the solver, whereas smaller ones in the range of minutes could indeed suffer from the increase in multi-processor management overhead.
The internal Nastran settings are stored in the plain-text file Netfabb installation folder\Nastran 2018\Nastran.INI. The directive NPROCESSORS determines the count of physical CPU cores available to Nastran, ignoring any virtual cores as provided by, for example, Intel® Hyper-Threading Technology. The default value is 2. You may want to benchmark Optimization Utility with projects of sizes and complexities typical to your requirements to find the optimal value for NPROCESSORS.
Read more about Nastran.INI in the Nastran online help here.