Frequency of cardiac muscle contraction compared to skeletal muscle contraction

In EK 9E Biology 2: Systems it states that

"The action potential of cardiac muscle exhibits a plateau after depolarization. The plateau is created by slow voltage-gated calcium channels, which allow calcium to enter and maintain the inside of the membrane at a positive potential difference. As a result, repolarization of cardiac muscle is slower and more frequent than that of skeletal muscle."

I'm not sure how they arrived at the conclusion that cardiac muscle repolarization is more...

Frequency of cardiac muscle contraction compared to skeletal muscle contraction