The Advanced Edition Companion, or AEC, is a companion to the core Labyrinth Lord rules, playing on the word "companion" as promised in the 1981 B/X rules. But instead of giving us rules for higher levels of play (we already got that in the core LL rules...basic, expert AND companion all wrapped up in one), this book gives us a B/X-style treatment of "advanced" rules. We get the classic 1e classes (Ranger, Monk, Druid, etc.), many more spells, many more magic items, and many more monsters.
And we don't get any more crunch. No combat segments, casting times, or weapon speed. This is basic play with advanced stuff, period.