That's because there is no low-speed mode. It changes based on a variety of conditions including how degraded the battery is (the OS tells you when it needs to be serviced once it's near the end of its life, 500 cycles), cold or hot temperatures, and the remaining battery life (low power mode decreases performance).
How did you come to this conclusion? My 6S was running in this “low speed mode” (as confirmed by Geekbench 4 results) even when the battery was at 100%, phone plugged in, placed at my desk at room temperature all afternoon. Only relevant variable seemed to be the battery age, and the results improved (along with subjective perceived performance increase) only when I recently had it replaced.
