Which of the following is NOT a cause of air compressor slowdown?

High oil levels in the air compressor system do not typically cause a slowdown in performance. In fact, proper oil levels are essential for lubricating components and ensuring efficient operation. While excessive oil may lead to issues such as foaming or contamination, which could theoretically impair performance, it does not directly cause the compressor to slow down in the same way that other listed factors would.

In contrast, a plugged aftercooler can hinder the airflow and reduce the compressor's ability to cool and compress air effectively, leading to performance issues. Low line voltage can result in inadequate power supply to the compressor's motor, causing it to run slower or stall. Lastly, a dirty or oily commutator terminal can affect the electrical connections and motor performance, thereby contributing to a slowdown. Each of those factors disrupts operational efficiency, whereas high oil levels, when they are not excessive, can be managed effectively without immediate adverse effects on performance.