While working on the DevTools profiler, I've noticed that durations are sometimes incorrect. This seems to be vaguely related to high/low priority state updates. Need to track this down more so I'm filing an issue for myself.
target.child = source.child;
  target.sibling = source.sibling;
  target.index = source.index;
  target.ref = source.ref;
  target.pendingProps = source.pendingProps;
  target.memoizedProps = source.memoizedProps;
  target.updateQueue = source.updateQueue;
  target.memoizedState = source.memoizedState;
  target.firstContextDependency = source.firstContextDependency;
  target.mode = source.mode;
  target.effectTag = source.effectTag;
  target.nextEffect = source.nextEffect;
  target.firstEffect = source.firstEffect;
  target.lastEffect = source.lastEffect;
  target.expirationTime = source.expirationTime;
  target.childExpirationTime = source.childExpirationTime;
  target.alternate = source.alternate;
  if (enableProfilerTimer) {
    target.actualDuration = source.actualDuration;
    target.actualStartTime = source.actualStartTime;
    target.selfBaseDuration = source.selfBaseDuration;
    target.treeBaseDuration = source.treeBaseDuration;
  }
  target._debugID = source._debugID;
  target._debugSource = source._debugSource;
  target._debugOwner = source._debugOwner;
  target._debugIsCurrentlyTiming = source._debugIsCurrentlyTiming;
  return target;
}