Yomaira told Diana that sometimes she has trouble understanding things in class because at times she speaks to fast. This sheds light on the fact that students students might feel comfortable expressing this to their teacher, but not necessarily to their manager. You can always ask them: Do you understand? or even "Am I talking to fast?". They'll be glad you brought it up!
By the way, Yomaira's doing an exemplary job in class. Her writing is nearly perfect and she can always produce our grammar point at by the end of class.