Psyringe
Scout
Just to be clear. On the slower speeds (epic and marathon) techs take longer to discover. Unit creation and movement remain basically the same? Or is it just movement? What about workers? Does it take longer for them to build improvements?
The rule of thumb is: Almost everything (tech cost, unit cost, building cost, worker job duration, golden age duration, etc.) scales by the same factor, to keep all game speeds balanced. However, in marathon speed there's one exception: The hammer costs for unit get a 33% rebate after scaling, i.e. they cost just 67% of the amount that they would cost after scalng by the same factor as techs, buildings and all the other stuff.
The actual mechanics are a bit more complicated (there are some things that don't scale, like the 10 turns minimum peace duration), but as a rule of thumb I found the above to work well.
The main advantage of slower game speeds is that unit movement does never scale, so with slower gamespeeds, units can cover more ground because they have more turns to move. This makes it easier to wage war, and easier to recover from wars.