Animations broken in protected watches

I have a watchface which uses the image widget to create a sprite, and then the Image Selection field is set to one of the three frames, switching between them every 1/8th of a second.
It was all fine until I exported it as a protected watchface, then testing this file results in a large number of skipped frames. Other animations (moving objects based on the number of milliseconds) still appear to be at 50fps.