We had been costing the slide as if it were fixed cost, regardless of LMUL. This results in significant under-costing of high LMUL inserts and extracts.
Note that the result of this patch is actually an over cost. After my recent D158460, we really should be using index when known to adjust the cost. I'm deferring that to a separate change as there's an analogous change to be made on the insertelement side first.