Learn Spanish (Latin America): Curiosamente, su desempeño es más superficial de lo que parece. - Curiously, his performance is more superficial than it seems.

In this position, curiosamente acts as a speaker attitude marker. It modifies the entire sentence rather than a specific verb. Instead of saying how an action was performed, it tells the listener your opinion about the whole situation, meaning it is curious that his performance is superficial.

Why do we use de lo que instead of just que for the word 'than'?

Because the comparison links to a new conjugated verb (parece). When you compare an adjective (superficial) to a full clause with a verb rather than to a simple noun, you must use de lo que instead of que.

Does desempeño refer to a performance in a play or a movie?

No, desempeño refers to how well someone does a job, a task, or how a machine functions (like job performance or academic performance). If you meant a theatrical or artistic performance, you would use a word like actuación or presentación.

Could su refer to someone other than 'him' in this sentence?

Yes, because su is the possessive adjective for the third person and formal second person, this could equally mean her performance, their performance, or your (formal) performance, depending on the surrounding context.