What? 91 octanes? 94 octanes? That is probably why stock American cars perform better in Europe... the lowest I can find around here is 96 octanes, I usually choose 98 octanes or 100... depending how much money I have in my pocket LOL!
Look closely at your fuel pump at the station and tell us if it's rated RON, MON, or R+M/2. In the states, it's R+M/2 (RON plus MON divided by 2). I'm guessing that you're seeing RON ratings which are much higher.