It's possible that this is "Movement" (2011) by Nancy Fulda, first published in Asimov's, March 2011 and collected in Nebula Awards Showcase 2013 (ed. Asaro).
The story is told from the daughter Hannah's perspective; the first scene is a neurological specialist describing a potential treatment to her parents:
Behind me my parents and a specialist from the neurological research institute are sitting on folding chairs they’ve brought in from the kitchen, quietly discussing my future. They do not know I am listening. They think that, because I do not choose to respond, I do not notice they are there.
The neurological specialist is offering a chance at "normalacy:"
"It's a matter of trade-offs, Mrs. Didier. The brain cannot be optimized for everything at once. Without treatment, some children like Hannah develop into extraordinary individuals. They become famous, change the world, learn to integrate their abilities into the structures of society. But only a very few are that lucky. The others never learn to make friends, hold a job, or live outside of institutions."
"And... with treatment?"
"I cannot promise anything, but the chances are very good that Hannah will lead a normal life."
Hannah is not "classically" autistic, she is diagnosed as having "temporal autism" which primarily affects her time sense (both perfect in measuring and also somewhat personally unbound from it), other autistic behaviours arising from that.
I have pressed my hand to the window. The glass feels cold and smooth beneath my palm. It appears motionless although I know at the molecular level it is flowing. Its atoms slide past each other slowly, so slowly; a transformation no less inevitable for its tempo. I like glass — also stone — because it does not change very quickly. I will be dead, and so will all of my relatives and their descendants, before the deformations will be visible without a microscope.
I feel my mother’s hands on my shoulders. She has come up behind me and now she turns me so that I must either look in her eyes or pull away. I look in her eyes because I love her and because I am calm enough right now to handle it. She speaks softly and slowly.
In the story her internal monologue is rational and coherent, but she is almost non-verbal, and the story tries to show how she experiences the world differently.
The word they have made for my condition is temporal autism. I do not like it, both because it is a word and because I am not certain I have anything in common with autists beyond a disinclination for speech.
But they do not know how to speak on my time scale. Their conversations are paced in seconds, sometimes in minutes. It is like the buzzing of mosquitoes in my ears. I need days, sometimes weeks to sort my thoughts and find the perfect answer.
Hannah decides that she doesn't want to be "normal."
I do not want to live small. I do not want to be like everyone else, ignorant of the great rush of time, trapped in frantic racing sentences. I want something else, something that I cannot find a word for.
I pull on Mother’s arm and tap at the glass, to show her that I am fluid inside. As usual, she does not understand what I am trying to tell her. I would like to clarify, but I cannot find the way. I pull my ballet slippers from the rustling paper bag and place them on top of the information packet left by the neuroscientist.
"I do not want new shoes," I say. "I do not want new shoes."
