Music, if it is to be perfect, can't be perfectly timed. A perfectly timed musical composition may sound mistimed to our stupid human brains, especially if it's synthesized.
Some people have spent large parts of their childhood noting the effects of varied perceptual attack time — the lag between when the note begins playing and when we feel the note has "hit." They're the ones that grew up in the Casio decade, when thousands of sweetly hopeful parents went out and bought their kids little toy keyboards hoping to stimulate musical genius, only to have those kids engage the automatic "rumba" beat and play "Ode To Joy" as a series of meows.
Somewhere along the way, we kids figured out how to make different keys make sounds like different instruments, so we'd go up the scale playing a note on the piano, a note on the harpsichord, a note on the violin, and so forth. That sequence sounded even odder and cheaper than "Meow To Joy," and it sounded that way in part because of perceptual attack time.
Perceptual attack time does not necessarily mean the time it takes for humans to notice the sound of the note. Most humans hear a note well before they feel its emphasis in a piece of music. A close analogue would be the moment of the note's greatest intensity, but that's not quite it. The perceptual attack time, though it depends on a lot of physical factors, is about psychology, not physics. We know when a note lands.
And the note lands differently for different instruments. Violins and cellos make tones that rise, which means that their perceptual attack time can be at a few dozen milliseconds after the note starts. Drums and cymbals, on the other hand, have barely a millisecond before our brain feels they have hit. This makes sense. Drums couldn't be percussion instruments if they didn't register in our consciousness the moment they made a sound.
It's the instruments with the long rise that are tricky — with wildly varying times depending on their pitch and tone.
There's little chance this will screw up your next concert. The time lag is too small for that. But it does mean that a series of notes played on a series of instruments will sound badly-timed no matter how perfect the timing is. We don't feel a note from violin at the same time we feel the notes from a piano or a clarinet. Any keyboard playing those notes needs to compensate for the human mind rather than the acoustics of the room.