This is an orange pi 5 without cooling. Perhaps it can’t reach the maximum because it has a thermal throttle? The CPU temperature is 42 ºC.
I’m using the same program in another computer and it uses more CPU and in a more consistent way. Without the lines being all over the place.
If I remember right my pi doesn’t throttle until around 55. 42 seems low to be getting performance reduction when they’re rated into the 80s.