Usually in 2D physics engines like JBox2D, if the user wanted to make a simulation run in "slow motion", you would just decrease the value by which the simulated world iterates by, for example normal movement at 60fps, the step would be 1/60 if the physics world is stepped forward every frame.
But for the SKPhysics class in the sprite kit in xcode 6 using swift, I only see the property called ".speed" which when I decreased, only led to a jittery mess, that updated the physics bodies once every 10 frames.
Is there a specific property, or trick maybe to get the SKPhysics world to iterate forward in time by smaller increments without gross glitchy movements?
The only way to slow down the simulation in sprite kit is to use the physicsWorld.speed element.
But if you are trying to change only one or two sprite speeds, try the velocity element of the sprite:
you can directly change the velocity(speed) of your node or sprite through its physicsBody.
Hope it helps :)