Speed

Speed is an Attribute which every unit has. It measures how fast a unit can move and it's represented as a shoe with wings on it. The advantages of speed varies with where it's used.

Strategy

Ground units in ages before gunfire could dodge arrows flying at them by moving out of the way when launched, this is easier with mounted units, as they typically start with 16 units of speed. There is a tactic that involved mounted Archers contently moving to and from the enemy in hopes of evading as much damage as possible and causing the most damage possible. When guns are invented, they never miss and hit their target instantly, which renders dodging useless.

Air units benefit from speed as the faster they go, the less Fuel is needed to go to their destination, therefore increasing the 'range' of your air units. Airplanes also evade some damage from Anti-Air because the faster the plane is, the faster it can fly pass (or retreat) the AA, the slower, the more gunfire (or Missle) it will take, damaging the already fragile airplane.

Note: this isn't an excuse to not seek out and destroy as much AA as possible before sending your air in.

All units benefit from increased Speed in that they reach their destination sooner.

The Speed Glitch

The Speed Glitch is one of the few legal glitches that can be performed in the game. The glitch involves using your army (of slow units) with at least one fast unit, such as mounted units (Flying units will not work). The fastest unit in your selected group will have it's speed 'shared' with the rest of the group, syncing the whole group. This group is one of the most commonly used glitches, and new players may not even be aware that they're using the glitch themselves unintentionally.

To perform, select all of your army you wish to send one, and have at least one fast unit, move your army to a destination by HOLDING down the right mouse button and releasing it (a yellow arrow should appear). This is clear when you see a bunch of siege units move as fast as your one knight.