The firm's own performance evaluations never indicated that customers had difficulty understanding her. You qualify for a raise in all categories. The important question in this case is not whether there was intentional and conscious discrimination, but whether or not Ms. Chow's accent interfered with her job performance. Clearly, it didn't. Any assertion that Ms. Chow could not comprehend or be understood by the company's clients because of her English is totally undercut by the company's own documentation, which showed that even under the new supervisor she exceeded her production goals.
Evaluations are often subjective, arbitrary and susceptible to unconscious biases.
For example, many in our society are more willing to tolerate French and British accents than Chinese or Spanish accents. The law requires that accents be evaluated objectively, not based on the uninformed gut reaction of a supervisor, acting without any evidence of an actual problem with work performance.