I mean (and I understand all this hasn't got the pretention of being 'objective'): "efficient" as in direct and to the point, without any unneccesarily long intro or repetitions in general, but which still stands out for its melodic and structural wealth.
Personally, I think if your going down the path of being worried about repetition then that rules out the vast majority of music, almost all of Iron Maiden's music.
I think some Prog music tries to cut out the repetition but carry a melody and song along a cohesive journey. But then, when bands do that, they get accused of being bloated, boring, indulgent etc. For example some people like "The Red and the Black" but criticise the instrumental section which goes away from the repetition and they claim that this instrumental is bloated and makes the song "unnecessarily" long.
It's impossible to please everyone. I certainly do think that many of us are way too critical when it comes to new music presented by bands we love. We just accept the old classics as the are because they were perhaps penned in a time where we weren't following. We take them as they are. But when new music comes along, we have hopes and dreams that a band will go in a certain direction, and when they don't we may get a bit disappointed.
I was quite happy with the direction Maiden took for AMOLAD and TFF and was a bit disappointed in TBOS for it reverting back to the old Maiden formula. But hey, just gotta keep an open mind, many of the songs are really, really good on TBOS. I think the only way to create a "perfect" album is to create it yourself. Because only you know, or define perfection as it is relative for you.
I certainly don't give points for "efficiency" in music. I don't heed the terms "bloated" or "indulgent". I love guitars, I love it if Maiden indulge in guitar melodies, instrumental pieces and solos.
Maiden certainly do use repetition well (IMHO). Although perhaps they have been guilty of making almost every chorus a repeated one liner (at times). TBOS doesn't have that aspect to it.
After being made to get used to long (7+ minutes) and unfortunately often bloated tracks that Maiden had got the listeners accustomed to on a very frequent basis since The X Factor
My one criticism of The X Factor is the disparity in volume. Many times it has a very quiet intro. If I leave my MP3 player at a particular volume then it is almost as if there is a minute or two of silence before the song is audible. Otherwise I am forced to change the volume all the time.
I really like the Prog aspect, I really like the longer songs, I struggle to understand when other people state that they want the shorter songs. But to each his own. Maiden appeal to a great variety in musical tastes.