This is a good example of where better punctuation would have helped readers: "He had a very weak -- yet a very violent -- performance, unlike his predecessor." The emdashes (--) mark the interruption of the sentence for the side remark "yet a very violent". "Weak" and "violent" both refer to the performance.

Did you copy this accurately? If you did, I would not worry about it, as it is very poor English. It reads like something badly translated by someone with English as a second language.