It is a custom gamerule. When using the /gamerule command to modify the name of a gamerule that isn't default, a custom inert gamerule is created instead. "speed" is not a default gamerule.
Custom gamerules do nothing on their own, requiring the use of command mechanisms to detect the value and manually do something with it. If you do not have any command mechanisms that use it, then it does nothing at all.
Best Answer
Custom Gamerules makes you add other rules. This is an example:
That is one of the custom gamerules. Tutorial on how to make a custom gamerule: http://www.minecraftforum.net/forums/minecraft-discussion/redstone-discussion-and/2353772-tutorial-make-your-own-custom-gamerules