What percentage of BL recruits simply can't hang?

To preface, I am not biglaw, I am shit law, so asking from the outside.

What I am wondering, is how many biglaw summer recruits and/or new associates simply lack the skills necessary to perform the work? Or are incapable of developing those skills?

The recruiting method, as far as I know, is fairly simply. Gather students from the top schools, or the top students from lesser schools, because they are most likely to achieve in the biglaw environment. But I'm wondering, how many of these recruits, despite their stellar background and the work it took to get way they are at, simply do not have the mental capacity to perform?

My guess- The number can't be high, these are high achievers that are probably capable of doing any kind of work at a high level. With that said, there's no doubt at least one biglaw associate has been fired due to incompetence.