A while ago I added some debug rendering to my engine that shows me all the physics bodies in the scene, and colours them based on their state: blue represent totally static bodies (scenery), green represent bodies in motion (being solved that frame), and red representing bodies that are currently sleeping (auto-disabled). A quick look at the bodies representing my doors showed them green all the time - meaning that the physics solver is recalculating them every frame even though they're just sitting there (because there's no animation playing).
While my first thought was that I could just leave them asleep all the time given they didn't actually need any simulation, I soon realised it's important to make sure they're made active when they're moving, or any auto-disabled bodies that are touching them won't react when they start moving (imagine a character hanging in the air when a trap door drops open beneath him - not good).
The first thing I had to do was let the physics engine know that these doors don't need any motion simulation - they're pure animation (i.e. no gravity, friction, momentum, etc). My physics engine allows you to lock specific axes on any given body (so for example on a see-saw, you want to lock translation in x y and z so it doesn't slide anywhere, then lock rotation around y and z, leaving it free to pivot around the x axis) - so for any animated scenery (like a door, or a lift) I just lock all the axes. The physics engine now knows this body is only there to interact with other bodies - it doesn't actually need any simulation itself.
Next, I just added some auto-disable code to wake the body up when it receives any motion from a controller (e.g. the animation) and put it to sleep whenever there's no motion present.
Here's the result - the first shot shows a door sitting full closed (notice that it's red, so it's not taking up any cycles), and the second shows it automagically waking up to open (in green). I can't tell you how many times this coloured physics feedback has saved me hours debugging stuff!