Define inefficient.
For this kind of ready-made modules, 80% would be about what to expect, and could hardly be qualified inefficient. Finding something as cheap (which I suppose it is) with better efficiency may take a while.
Designing it yourself - yeah, doing better than 80% if you have no preliminary experience with that, good luck!
Now if your module has significantly worse than 80% efficiency (are you sure? how did you measure it?), you probably either got ripped off, or you're not using it properly.
Lastly, just in case you haven't really measured the efficiency, but are basing your impression on the fact that the converter gets "hot": well, if it has say 80% efficiency in your conditions, it's still 20% loss. For the worst case, going from 12.6 to 5V @8A, that's a dissipation of 12.16W. Certainly enough to get a small converter very hot if it's not properly heatsinked.