Remember conservation of momentum. The only way the machine can absorb part of the impulse is through friction, heat, and by redirecting the existing chamber pressure after the bullet has left the barrel.

Remember the human body. Magnitude matters much more than duration. Extending the time of impulse by implementing a slide lessens magnitude, the areas under the impulse curves roughly equivalent.

I'm going to apply the above to answer your questions to say it again :)

Does the slide absorb any significant amount of energy?

For a properly functioning, modern, and typically-designed pistol and a status quo definition of "significant", the answer is: No. That's not what it's designed to do. But, energy can be dissipated slightly if the pistol is compensated: a redirection of chamber pressure from near the end of the barrel, upwards, counter the torque component of the recoil impulse.

What's the math on this, say the dissipated energy in a semi auto VS revolver using the same round?

It's not quite a good question. The maximum force during the impulse is what a human cares about when analyzing a slide. That's what'll effect accuracy of the next round and how sore your hands will be in the morning.

If minimization of total impulse is what's being analyzed then one would want to compare rifles. Rifles have larger rounds, longer barrel length thus more time to use chamber pressure to mitigate recoil.

You've good questions for coming into the middle. Go to the beginning: rounds and various types of actions, rifleman 101. Come back to the hard science.