Duty cycling
Keeping a device asleep almost all the time and waking it briefly at fixed intervals, because the only thing cheaper than a sleeping radio is one that's switched off.
In a battery-powered sensor the radio is nearly always the biggest consumer. So the device wakes, measures, transmits and goes back to sleep — perhaps a few milliseconds every ten seconds, a duty cycle well under one percent. Battery life is then set almost entirely by sleep current and wake-up frequency, not by how fast the CPU is.
Two complications follow. Transmitter and receiver have to be awake at the same time, and since their clocks drift apart the receiver must wake a little early — a guard time that eats into the savings. And sometimes it's the law: in the EU 868 MHz band, ETSI rules limit many transmitters to one percent airtime, which is why LoRaWAN devices can't chat as much as they'd like.