Consider this statement from the assertion spec.
Programmers developing applications for resource-constrained devices may wish to strip assertions out of class files entirely. While this makes it impossible to enable assertions in the field, it also reduces class file size, possibly leading to improved class loading performance. In the absence of a high quality JIT, it could lead to decreased footprint and improved runtime performance.
Assertions disabled, compared to the absence of assertion code, could degrade performance. Is that right?
Does your favorite exam prep book say assertions disabled do not cause any overhead?