On a single-core/single-thread Windows system the problem would be avoided because when the waiting thread is signaled it gets a priority boost. With just a sin

On a single-core/single-thread Windows system the problem would be avoided because when the waiting thread is signaled it gets a priority boost. With just a single core this boost means that it gets to run instead of the other thread. With multiple cores it gets to run as well as the other thread. Making the locks fair or occasionally fair would solve this problem, but perhaps cause others. I think that an appropriate CPU-saturating background task might have made my machine more responsive, which is deliciously weird.