Solution time goes up when elements change shape and the characteristic length goes down. Plot equivalent strain to find the location in the model where the highest element distortions have occurred.
The solver has a feature called Erosion, which removes elements that are highly distorted or have exceeded the material failure limit. Under Analysis Settings, Erosion Controls, you will find the default settings are to remove elements whose strain is larger than 1.5 (150%). There is also a setting to remove elements on Material Failure, assuming the material has failure defined.
Are you using a material from the Explicit Dynamics library, such as one that has an EOS material definition? Have you changed the default erosion controls? Try using a material from the library and change the setting to remove on Material Failure.