Pretty sure the A chipsets can change frequency on demand (at least since A7) for power and thermal regulation. Mind, the point of the big.LITTLE-like CPU configuration on the A10 Fusion and A11 Bionic is power savings. The iPhone 7, 8 and X probably use the low power cores 70-90% of the time.
The default setting should be CPU throttled almost all the time only ramping up when there's high demand otherwise, you'd kill battery unnecessarily much sooner. Checking email, listening to music, watching videos or reading ebooks don't really require gobs of processing power.
Iirc, as a point of comparison, factory settings on my Windows laptop when on battery is min CPU state 5% and max CPU state 100%. The power saving profile limits max CPU state to 50%.