If you cannot coarsen your mesh, there are other techniques available to reduce computational time in explicit dynamics analysis. For instance, you can employ mass scaling with the corresponding parameters available under Analysis Settings in Workbench LS-DYNA.
In addition, if the plate is much stiffer than the ball, you can assume the plate to behave as a rigid body.
I recommend you to see the course below for a more detailed description of available techniques to lower computational time:
Explicit Dynamics Theory - Ansys Innovation Course - YouTube