I believe Humminbird adjusts the sonar ping rate automatically based on depth and sonar frequency. It automatically adjusts to the highest workable ping rate, and is not user adjustable.
Chart speed is a setting that determines how quickly the sonar image scrolls across the display, separate from ping rate. As mentioned, it’s typically set to the same or +1 to your speed for the cleanest images, and you can increase it to elongate or exaggerate them.