I remember how slow I was in the beginning, but I find myself getting faster and faster. A lot of times, when I'm trying to figure out how fast I'll need to be at an event, I can't go by what the client says. They may think hordes of people are showing up and then they don't. Or they'll estimate low and it will be the complete opposite!
For events where they're expecting tons of people, I usually take a sign and only list my quickest faces. I had an event like that not too long ago. It was a fundraising event. I get there, put out my sign and am ready to whip out beaucoup faces. Everyone seems to be picking the quickest of my faces listed on my "quick face" board. Hmmm... so I'm blowing through all these faces and I realize, I still have lots more time to go and there doesn't seem to be that many more kids left. Woops!
Before showing up, the guy had told me he might have me stay an extra half hour if needed. I thought, well that blows that opportunity.

Anyway, I started slowing way down and thankfully a couple of the kids picked one of the faces from my board that I could take my time, add more details, etc.
How many faces per hour do you quote? I have down that I can paint approximately 10 - 15 faces per hour, depending on how complex the design is that they choose.