The amount of total turns in the game has been radically rebalanced for BTS. Previously Epic had almost 2x the turns of normal, but only 1.5 x the build, train and research costs. Marathon had approx 2.5x the turns but 3x the research, build and 2x the train costs.
Now:-
Normal :- 500 turns 1x Research, Build and Train (where train is building units.)
Epic:- 750 turns 1.5x Research, Build and Train.
Marathon:1500 turns 3xResearch,Build, 2x train.
So now they are "true" speed multipliers, with the only disparity being units built at 2/3rds the cost on marathon.
Of course, the game is not linear, and small effects can have large changes (because of its nature of settle , expand, Civ tends to grow almost logarithmically at points, but that doesn't need analysis here
)
Anyways, there are the BTS gamespeeds for comparison.
N.B. Sorry I forgot about quick speed, but if you like units becoming obsolete before they can even finish in the build queue, then Im sure thats for you.