That seems a little reductive to me, since "needs more time" could mean many different things, ex:
1. (Overall rate) From the same starting-point, this child will always require 2x the time/effort to learn than expected.
2. (Overall offset) This child's thresholds for becoming able to learn many skills at the normal rate tends to be 6 months later than expected.
3. (Specific rates) For a particular task this child learned slowly no matter how old they were.
4. (Specific offsets) For a particular task this child learned slowly, but when stopped trying and revisited it X months later they had no problems.
Of those, it sounds like you're assuming the harshest, first version.
I'm no child developmental psychologist, but I'd wager that versions 2,3,4 are more frequent than most people would assume, since they take more work to distinguish and the universe tends to be messy.
1. (Overall rate) From the same starting-point, this child will always require 2x the time/effort to learn than expected.
2. (Overall offset) This child's thresholds for becoming able to learn many skills at the normal rate tends to be 6 months later than expected.
3. (Specific rates) For a particular task this child learned slowly no matter how old they were.
4. (Specific offsets) For a particular task this child learned slowly, but when stopped trying and revisited it X months later they had no problems.
Of those, it sounds like you're assuming the harshest, first version.
I'm no child developmental psychologist, but I'd wager that versions 2,3,4 are more frequent than most people would assume, since they take more work to distinguish and the universe tends to be messy.