Your house has a flow rate—how fast water moves through pipes. If you have a big soaking tub or a sprinkler system, you need a softener that can keep up.
Check your water meter or time how long it takes to fill a 5-gallon bucket. A softener with a low flow rate will choke if you try to shower while washing dishes. Chaos.
Quirky fact: some softeners are so slow they’ll make your washing machine hiss in protest. Don’t be that person.