My Grohe LadyLux3 kitchen faucet's flow rate has gradually slowed in the 4 years since we installed it, so that now it's flowing at 0.7 gallons per minute, which is less than half its rated flow rate of 1.75 gallons per minute. The flow isn't even strong enough to keep the "spray" setting active without holding the button on the faucet head. When we first installed the faucet, the flow rate was close to the rated level.
I assumed that the problem was sediment buildup in the aerator so I replaced the cartridge in the faucet head. Surprisingly, this didn't fix the flow rate at all. I then tested the flow with the head removed completely, and the flow rate only slightly increased to 0.77 gpm-- still less than half of the faucet's rating.
The flow is the same regardless of hot, cold, or mixed-temp water. It's the same when just turning it on vs. leaving it flowing for a few minutes.
No other faucets in the house have shown a similar decline, so it seems unlikely to be a house-wide change in water pressure.
The change has been gradual-- it's not like one day suddenly the flow was cut in half.
The valves to both the hot and cold water under the sink are fully open.
We haven't made any plumbing-related changes since the faucet was installed.
Any ideas for how to get the flow rate back to normal?