We currently use a mixture of instant hover effects and 150ms hover effects.
Using a non-zero duration for hover effects generally produces a feeling of lag, especially during fast mouse cursor movement. When moving the cursor fast, using a fade for hover effects delays the "feedback completed" time in response to the cursor to an extent where the animation can barely progress before it needs to fade out again, producing the feeling of an unresponsive interface. Additionally, there's non-trivial implementation cost to adding hover animations & keeping them all consistent with each other is a hard task. Hover effects are our most common effect, as virtually every control has a hovered state. All the Behaviour on, the Transitions, and in the more complex cases, tens of lines of C++ scenegraph code (and hundreds of lines of QStyle) add up to a lot of easy-to-remove complexity in our codebases for an unappealing visual effect.
Therefore, I would like to propose that we move the controls still using hover animations to instant hover animations to first and foremost remove the sense of lag pervading some of our hover effects, secondly to increase visual consistency, and thirdly to remove the surprisingly large amount of code dedicated to implementing something that worsens the user experience.