(as) round as Giotto's O

Quickly yet perfectly done. The phrase refers to the Italian artist Giotto, who is said to have promptly drawn a perfect circle to demonstrate his artistic abilities for Pope Boniface VIII. Wow, I can't believe you wrote this paper in one night—your writing is round as Giotto's O. I can't believe you painted the whole room last night—and that your paint job is as round is Giotto's O! Susie's last-minute business proposals are always round as Giotto's O, so we're really lucky to have her on the team.