Snapshot 14w17a;Customized world loading is too slow

Hello. I have Log in to report a bug, because i have this bug:In Minecraft snapshot 14w17a, there is Custom world type. So i settled everything, and i tried to Create the world. But the result, it does not Loads the world that it Customized. Please fix or tell me!
PS:I have Force quit the game, and didn't Reopened the game. If does work well if reopened, please tell me.

Thank you for reading. Sorry if it is too long.