> Using them, one could travel in less than a second between distant points in our galaxy. A second for the observer that goes through the wormhole. It would be tens of thousands of years for somebody looking from the outside.
Unless the energy requirements are lower, it sounds like this isn't much better than just traveling close to c (assuming you had adequate shielding).
The stepping back in time would make it a causal loop if you were able to observe something happening in the past (light traveling from the source) from the exit side, then go back through the wormhole to change this.
To prevent this, the observed wormhole traversal time would need to be slightly faster than the speed of light, because then you also couldn't go back far enough to change something you already observed (assuming the "time" here is symmetrical to the time it takes going in the other direction).
However there are other problems: What happens when I have 2 matching wormhole pairs, each exit being located to the entry point of the other. If I step back through the exits again and again can I go back in time as far as I wish?
The wormhole sends you back in time, but not far enough back to arrive at your point of origin before you left.
If you could treat the wormhole like a magic door that takes you across space, then two people looking at each other through the door wouldn't see anything unusual. One would be in the distant future, by the galaxy's clock, but not by the wormhole's.
So going through in one direction takes you into the future, and the other takes you into the past, but it's a constant offset. The size of the offset depends on how you moved the other end to where it is.
It's true that with multiple wormholes you could try to create a loop, though. That would probably fail due to virtual particular loops.
Unless the energy requirements are lower, it sounds like this isn't much better than just traveling close to c (assuming you had adequate shielding).