Flow rate is how fast the water passes through the softener, and it matters because you don’t want your shower to become a sad drizzle. For a typical home, you need at least 10 gallons per minute (GPM). If you have a big family, aim for 12 to 15 GPM. A system that can’t keep up is like ordering a fire hose and getting a drinking straw.
Also, check your pipe diameter. If your main water line is 1 inch, don’t choke it with a 3/4-inch softener—it’s like putting a small funnel in a big jug. Your water pressure will thank you.