-What are the elements and factors to consider when determining the speed of a given strategy?
Knowing the answer to this question at the beginning of the game can often greatly impact the outcome. If there are two viable strategies on the board, but Player A knows which is faster when Player B doesn't, Player A has a better chance of winning, assuming he/she plays the deck cleanly. Any input on figuring out the approximate speed of one deck compared to another?
you could just aswell ask "how do i play correctly". understanding strategies is like over half of the game, you can't just give a single answer to that.
I quite disagree on that statement. It's more like tactics have a huge impact. I can see the right strategy on more than half of all games - yet I cannot apply it, because I'm lousy at tactics. The questions, when to pick up which card and how many of each, are way harder than "Is big money better than the Engine on this board?".
Anyways, IF you are making wrong decisions on your strategy, you're likely to lose the game no matter how well you play that bad strategy.
It's important to know, that gaining 5 Provinces by turn 13 is NOT winning the game. An Engine, that produces a lot of coins (or reduces prices) and buys can easily pile out duchies or alt. VP cards in 1-2 turns. It's more important to know: "How long would it take for the slog/BM to gain 50% of all VP available?". If there is no alternative VP at all, you can get 8x6+8x3+8x1 = 80 points plus up to ten curses to deal eventually. Gaining 50 Points on such a board means game over. However, the game might end on a 3-pile and it might end on Provinces. As a rule of thumb, I would say, if you're building an Engine and you are 3-0 behind on Provinces, you probably shouldn't buy Provinces unles you can get more than one. You'd better try to catch up a little bit before getting Provinces in order to not suicide-end the game.
Certain decks want to green early, whereas others like to green late. What factors determine when a given engine should green?
well, I can't give a clean answer here either, but generally there are two things to consider. #1 how fast does the engine gain strength whille you are building, and #2 how much do green cards hurt. an engine with hunting party will gain strength somehwat linear, an engine with highway has more of an 1/x character, meaning it will explode at a certain point. consequentially, you want to build a lot more for a highway engine.
#2 - well, you can take the exact same examples. a hunting party engine does care about green cards, but not that much, once you have one province on your hand, it willl just roll over the others anyway. a highway engine usually chokes really fast, because highways don't draw.
Also, if I'm a bit behind mid-game, should I pick up that Duchy with 5, or continue building?
Having a plan of how you want the game to end, e.g. on a megaturn for you, or maybe on 4 consecutive turns where you buy all your VP cards, really helps you. It's even better if you consider what it takes for your opponent to win the game and maybe deny this, but that's a level only few people have reached so far (and I am not one of those).
On your last question: What exactly does "behind" mean? "Behind on points" - if you've got the time, build some more. "Behind on building" - get something that helps you with green. Buying Duchies before Provinces is hardly ever a good idea, maybe in rushes and slogs, but not in an engine.