If you slow down the pace to help the "zero percenters" and only cover 80% of the material in the allotted time, the students who could have handled 100% of the material will be limited to 80%. And that slowdown still won't be enough to help the slowest learners much, so they'll still only learn, say, 20% of the material.

Right. In fact you can find studies to back this up.

In schools with blended ability levels, the worst students do better (pulled up by the higher students), but the best studends do worse.

Good if you are in the bottom, bad ify ou are in the top.

also spend lots of class time repeating background material students should already know.