The ADA makes it illegal for a private employer to discriminate against a qualified individual with a disability only on the basis of disability. Therefore, an employer is free to select the most qualified applicant available and to make employment decisions based on reasons unrelated to the existence or consequence of a disability.
For example, if two people apply for a typist position, one is a person with a disability who accurately types 50 words per minute, the other a person without a disability who accurately types 75 words per minute, the employer may hire the applicant with the higher typing speed, if typing speed is needed for successful performance of the job.