I reported on this earlier. On my 12, my water was so slow that the tankless heater kept shutting down because of over-temp. I measured the flow to be 0.6 gal/ min.
I found a one-way valve at the very end of the spigot hose; the end that attaches to the faucet, way underneath the sink. I do not know the purpose of the valve, but after I got it off, I tried blowing in the flow direction. It took some pressure to overcome the spring in it. This is what robs the flow. After removing it, the flow went up to 1.6 gal/min.
You probably won't recognize the valve. It is silver and barrel shaped, and looks to be part of the hose. Good luck finding it.
Gale & Hank- 2012 Berkshire 390BH