But chopping twenty points off an average person's IQ does not make him "intellectually disabled", just excruciatingly slow.

The usual boundary for "mentally disabled" is IQ 70. There are a LOT of IQ 90 people walking around, chopping off twenty points won't work well for them.

And a warning -- as was pointed to me recently IQ points are really ranks. There is no implication that a one point difference (or a twenty point difference) means the same thing in the 70 - 90 context as in the, say, 120 - 140 context.