Bullet sound with ... latency ?

If you're playing an mp3 for a sound effect, that will make it slow. Also if you're using Resource files or "preload audio data" turned off, there will be latency as the clip must load before playing. If you're not doing any of those things, are you running on Android? Unity has latency problems on that platform in particular and there's nothing we can do about it.

And depending on what's on your bullet prefab, using Instantiate instead of pooling the object could be causing lag as well.

Note that you don't need to write code for this usage. Put an EventSounds script on the bullet and use Enabled event to play the Sound Group.