LION-FS: Fast & Slow Video-Language Thinker as Online Video Assistant

Wei Li, Bing Hu, Rui Shao, Leyang Shen, Liqiang Nie
2/10/2026

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Abstract

First-person video assistants are highly anticipated to enhance our daily lives through online video dialogue. However, existing online video assistants often sacrifice assistant efficacy for real-time efficiency by processing low-frame-rate videos with coarse-grained visual features. To overcome the trade-off between efficacy and efficiency, we propose "Fast & Slow Video-Language Thinker" as onLIne videO assistaNt, LION-FS, achieving real-time, proactive, temporally accurate, and contextually precise responses. LION-FS adopts a two-stage optimization strategy: 1) Fast Path: Routing-Based Response Determination evaluates frame-by-frame whether an immediate response is necessary. To enhance response determination accuracy and handle higher frame-rate inputs efficiently, we employ Token Aggregation Routing to dynamically fuse spatiotemporal features without increasing token numbers, while utilizing Token Dropping Routing to eliminate redundant features, and 2) Slow Path: Multi-granularity Keyframe Augmentation optimizes keyframes during response generation. To provide comprehensive and detailed responses beyond atomic actions constrained by training data, fine-grained spatial features and human-environment interaction features are extracted through multi-granular pooling. They are further integrated into a meticulously designed multimodal Thinking Template to guide more precise response generation. Comprehensive evaluations of online video tasks demonstrate that LION-FS achieves state-of-the-art efficacy and efficiency.

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Cite this paper

@article{li2026lionfs,
  title  = {LION-FS: Fast & Slow Video-Language Thinker as Online Video Assistant},
  author = {Wei Li and Bing Hu and Rui Shao and Leyang Shen and Liqiang Nie},
  year   = {2026},
  eprint = {2503.03663},
  archivePrefix = {arXiv},
  url    = {https://arxiv.org/abs/2503.03663},
  journal = {CVPR 2025 2025}
}

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