In North America the Aesop fable about the tortoise and the Hare is well known, and glyphs of a tortoise (turtle) and hare (rabbit) are understood to represent varying speeds.
Is this generally the case cross-culturally?
Perhaps in spite of Aesop's fable they might be understood, but only as long as the users are familiar with rabbits and turtles and their relative speeds IRL. You also have to keep in mind that some cultures hold various animals in either very high regard, or as despicable and unclean, and you wouldn't want that to influence the way your users experience your site/app/whatever-the-context-is. I don't know the specifics for all cultures + turtles and rabbits, so you'd want to do user testing for sure.
To give you a good example, RockShox suspension for Bicycle uses icon of a hare and tortoise to denote the speed of the rebound for their suspension.
Slower (tortoise) or Faster (Hare)
To answer to your question, I believe tortoise (turtle) and hare (rabbit) are understood to represent varying speeds cross culture.