I'll take a stab at it since no one else has.
Just because someone is competent does not necessarily mean he/she will perform well (think of a top-level baseball player who is drunk, on drugs, distracted by family issues, unmotivated, etc. and therefore has a terrible game or games).
However, competence is a prerequisite for good performance. Someone who is incompetent cannot perform well (except through dumb luck). Incompetence is almost a guarantee of poor performance regardless of desire, motivation, or effort (think of the average motorist pulled from the family car and offered $10M for winning their first NASCAR race with only 1 hour's practice).
Make any sense?