When you check on the Evaluate Optimal Number of Groups parameter, the tool will solve for 2, 3, 4, or up to 15 groups. This process can take a while to complete, especially when the Spatial Constraints parameter is NO_SPATIAL_CONSTRAINTS and the number of features is large.
Uncheck the Evaluate Optimal Number of Groups parameter and use some other strategy for deciding how many groups to create. Once you have identified the optimal number of groups for a fixed set of features, Analysis Fields, and particular Spatial Constraints, you will not need to repeat this evaluation step. If you change the Analysis Fields, Spatial Constraints, or Input Features, however, the optimal number of groups (as determined by the pseudo F-Statistic) might change. The spatial methods for the Spatial Constraints parameter solve much faster than the NO_SPATIAL_CONSTRAINTS option.