Like Mario said, clipping has to be calculated for all of the attachment images that are being clipped, and it looks like basically everything needs to be clipped on your character when they are in the ground (since the only visible part is the top of their head).
I believe clipping works by calculating new mesh vertices for each attachment, so it is done on the CPU and is expensive if a lot of attachment images (with lots of vertices) need to be clipped.
I would look into using a Sprite Mask (https://docs.unity3d.com/Manual/class-SpriteMask.html), to mask out the area that is considered 'below ground', and then use the "Mask Interaction" setting on the SkeletonAnimation component to make it 'Visible Outside Mask'.
When you use Masking/Stenciling, it's calculated on the GPU (it basically checks whether or not the individual pixel should be drawn to the screen or not, based on if that pixel location has had its mask/stencil value set) and is very efficient.