API GetUnitSpeed

Returns the movement speed of the unit. currentSpeed, runSpeed, flightSpeed, swimSpeed = GetUnitSpeed(unit)

• unit: : UnitId - The unit to query the speed of.

• currentSpeed: - current movement speed in yards per second (normal running: 7; an epic ground mount: 14)
• runSpeed: - the maximum speed on the ground, in yards per second (including talents such as Pursuit of Justice and ground mounts)
• flightSpeed: - the maximum speed while flying, in yards per second (the unit need to be on a flying mount to get the flying speed)
• swimSpeed: - the maximum speed while swimming, in yards per second (not tested but it should be as the flying mount)

The following snippet prints your current movement speed in percent: /script print(string.format("Current speed: %d%%", GetUnitSpeed("player") / 7 * 100))

As of 4.2, runSpeed, flightSpeed, swimSpeed returns were added. It seem you can also get target unit speed (not tested on the opposite faction). A constant exists: "BASE_MOVEMENT_SPEED" which is equal to 7 (4.2).