FlashPortrait, a novel approach in generative models, significantly accelerates long-portrait animation synthesis by up to six times while ensuring ID consistency. The method employs an end-to-end video diffusion transformer, utilizing a dynamic sliding-window scheme and higher-order latent derivatives to enhance inference speed without compromising the quality of the output. The research highlights FlashPortrait's ability to produce ID-preserving, infinite-length videos, demonstrating both qualitative and quantitative advantages over existing methods. This advancement addresses key challenges in video synthesis, offering a promising solution for efficient and consistent portrait animation.