Using version 4.1.0, I found out a delay before new tab pane is shown with JS behavior and fade effect.
This delay is the transition-duration value of the fade utility class.

Issue comes from the _activate method, line 144 in tab.js file.

There is a listener added to the previously activated tab pane for the transitionend event, but no classes are added to this element after that, so only the emulator can triggers the callback which activates the new one.

Have a look at the demo in the official doc.