As you found, when you use inverse clipping, it only clips the convex hull. This is by design and it is unlikely that we'll support inverse concave clipping because it would require a lot of CPU. It would require even more than than the normal (non-inverse) concave clipping that many users find eats up too much CPU when not used carefully.
In 4.3.62-beta and newer, clipping is much more efficient when the clipping polygon is convex (regardless of whether Inverse or Convex are checked). It is still a relatively expensive operation, but performance is better. Many users only need convex clipping so this should help.
The most efficient usage of clipping is 1) few vertices, ideally 3, the size of the clipping polygon doesn't matter, 2) few attachment vertices clipping, ie not the whole skeleton, and 3) the hull is convex.
If Inverse or Convex are checked and the hull is actually concave, then Spine has to do some extra work to compute the convex hull. Try to avoid that by keep the clipping polygon actually concave whenever possible to maximize performance.
