I've not met a person so majestic to rapidly undo the knot of a slinky. But I can tell you that my method of undoing slinkys are relatively easier.
I would suggest stretching it out carefully to see where the knot is in the first place.
Then if you want, you can slide your thumb slowly through the coil, aligning everything in place slowly working your way down to the knot.
And then once you make it to the knot, flip it or untangle it slowly as you work your way down, carefully pulling the overlays apart.