EDIT: It looks like TME finished writing before I did.
If you look at the content of the core-passage.css file referenced in the story format's CSS documentation you will see that two CSS rules named: .passage and .passage-in.
The first is applied to the Passage area and assigns an opacity related transition effect, the second is used within the process that handles moving from one passage to another. (eg the 'fade-in' effect).
You can use CSS like the following within your Story Stylesheet area to remove the passage's transition effect.
note: there will still be a very slight delay even after removing the CSS based transition effect.