1x

If you're playing it in single player, play what's comfortable.  For online multiplayer PvP games on the Diplomacy expansion, the speeds are set to all "Faster".

so he's asking about the update rate, which scales the speed of the whole game on the fly. some of the answers here are about the game speed options, which is not changable once you enter the game.

If I plan on playing at regular update rate throughout the game, I am planning on a longer game (even though all game speed options are set to 'fastest'). If I want a quick game, I drop the game speed settings to normal, then increase the update rate in the game. It can make for some slow finishes at times if the fleet sizes get very big and theres alot of planets and I am forced to lower the update rate

Normal speed is generally what I play on, unless it is multiplayer than its whatever the host has set.

1x

Super slow mode. aka -5.0

That way I can play SINS, Star Ruler, AI War, and SoTS. At the same time, or any combination of such.

Lately I've been at 2x, although if a really large battle (or more than 2 regular size ones) comes along, 1x. Late in the game w/ more than 5 AI.... 1x... gets too dang laggy for faster than that regardless of circumstances.