Slower physics simulation when bSubStepping and bTickPhysicsAsync are enabled

When both Substepping and Tick Physics Async are enabled, there appears to be an inconsistency between the part of the physics simulation that calculates the number of steps and the part that calculates the DeltaTime per step.

1. Open FirstPerson sample project.
2. Enable bSubStepping and bTickPhysicsAsync  in ProjectSettings.
3. Play in editor.
4. Shoot cube.

Then, the cube simulation is slower.