Use the Mass Scaling subnode to increase the density in small mesh elements in the domain.

When the density and stiffness are homogeneous, the smallest mesh elements limit the time step as the critical time step is proportional to the mesh element size h. Hence, a single mesh element with poor quality can severely degrade the step size and ultimately the performance of the explicit dynamic analysis.

Enter the Target cell time to automatically increase the density in such small elements.

Physics tab with Solid Mechanics, Explicit Dynamics selected:

Physics tab with Truss, Explicit Dynamics selected: