The ability to read unfamiliar music while playing is called sight reading, and generally speaking it's more of an exercise than a capability relevant to actual performance.
During performance, sheet music is like a cheat sheet during exam, a reminder of something you already know. Most of the music is in your head and hand. The sheet music is just there to prevent a memory slip. Some orchestral music can be awfully repetitive, and having the page there helps you keep track of where you are. You are not supposed to devote lots of attention to the page itself, (unless you didn't practice before the performance and decides to sightread on stage, which, you know, happens).
During performance, sheet music is like a cheat sheet during exam, a reminder of something you already know. Most of the music is in your head and hand. The sheet music is just there to prevent a memory slip. Some orchestral music can be awfully repetitive, and having the page there helps you keep track of where you are. You are not supposed to devote lots of attention to the page itself, (unless you didn't practice before the performance and decides to sightread on stage, which, you know, happens).