The range and airtime tradeoff
A higher spreading factor can make a message easier to receive in weak signal conditions, but it also takes longer to transmit. Longer airtime uses more battery and occupies the channel for more time.
A lower spreading factor sends faster and uses less airtime, but it needs a stronger link. A nearby device should not use the slowest setting when a faster one works reliably.
For many regions, SF7 is the fast, short-airtime end of the common range and SF12 is the slow, long-range end. The right setting depends on link budget, payload size, channel rules, and gateway density.
What beginners should remember
LoRaWAN performance is not only about maximum range. A healthy network balances range, airtime, gateway density, payload size, and message frequency.
This is why a deployment with more gateways can often improve capacity: more devices can use faster data rates because they have better radio links.