I reran the benchmarks with the fast path in _PyEval_SliceIndex() and the results are not conclusive. There is no clear indication that it improves the performance.

The problem is that most slices have no step, many slices are open ended on at least one side (i.e. usually have one or two fields set to None) and they are only used once, so integer index optimisations can only have a limited impact compared to instantiating the slice object in the first place.