I couldn't reproduce this bug. Perhaps the program you're using to display the GIF doesn't support the GIF frame delay feature? (E.g., Chrome seems to display them fine.) Feel free to open an issue on the repo if I'm missing something: https://github.com/edemaine/font-tetris

Ah, fwiw was using my iPhone for the test, so perhaps it’s just scoped to that. Not like that’s a use case you really need to support with this :)