I just did something that is going to piss off all the Europeans...
I had to soak my shower head in CLR all night to get the lime and calcium buildup off the head. After I dismantled it, I noticed it had "3.0 GPM MAX" written down the side of it. So, I said, "What makes it only 3?" I quickly found that there was a rubber choke-ring in it, 20 minutes later, that shit is out and it's now probably a 5 or 6 GPM system minus all the lime. Sweet water pressure, baby.