Just because a game doesn't apply to real-world logic in every physical possible way, doesn't mean it needs to toss everything out of the window. Games can only simulate various aspects of the world to various degrees; any and every game out there will sacrifice some sort of logic because the real-world rules don't make for good gameplay. Graal does happen to be on the farther end of the spectrum, but that doesn't mean it should break all the rules because it has broken others.

The fault here isn't that the turtle is slow, it's that these aspects of the game have been very poorly balanced. If the turtle is slow, mounts like the snail should have been slow too. If the turtle is slow and a two-player mount, but a rhino is fast and also a two-player mount... yet cheaper, then there should have been better balancing there as well. The Rhino should have been more expensive, for the turtle should have an incentive for being more expensive than another mount that is cheaper AND better.

However it's probably too late to fix these issues because it'd require nerfing items players have already paid for and in my opinion that's not fair unless they are compensated.