What's optimal Depends on alot of factors. If all you care about is finishing the game as soon as possible the optimal speed is of course "online".
On the other side the game itself is balanced arround the normal speed, so the more you move away from normal speed, the more unbalancing these factors will be. This can for instance be factors like Balance of movement points against Production and Science speeds.
On online speed (which is double as fast as normal) you will in real terms lose double the turns compared to normal speed if you don't settle at the first turn.
All other kinds of unit movement, exploring and warfare will also in real terms be in half the speed of a normal speed game (when playing online). This effects a number of things like speed of exploring, speed of getting Eurekas/inspirations dependent on exploring, relative time to move Builders and settlers arround, relative time to move troops to an enemy during war etc.
In my opinion, and the developers opinion, the game is best balanced arround normal speed. Ergo; the more you move away from normal speed the more inbalances.
My conclusion: I prefer normal speed or epic for singleplayer. For multiplayer I prefer fast speed (for the extra speed) but I try to avoid online speed. Online speed is more tolerable on the smallest maps though, where relative movement speed means abit less in warfare.
