Iphone 6s not working good

I bought an iphone 6s at aventura mall 10 months ago but it does not work good. It is very slow and some app close without i ask for this. I would like to know if someone also have this problem cause i do not know how to solve. I made contact with apple 7 times and they told me need to reinstal system all the times. I made it 5 times and did not work. They told me to send to repair and i made it. For my surprise i received it yesterday and touchscreen is not working. I receive it worse than i sent. So... i do not trust their repair. Does anyone know how to proceed to solve that?