NormFit: A Lightweight Solution for Few-Shot Federated Learning with Non-IID Data

Vision-Language Models (VLMs) have recently attracted considerable attention in Federated Learning (FL) due to their strong and robust performance. In particular, few-shot adaptation with pre-trained VLMs like CLIP enhances the performance of downstream tasks. However, existing methods still suffer from substantial communication overhead, high local computational demands, and suboptimal performance under non-IID user data. To simultaneously address all those limitations, we propose NormFit, a lightweight solution that selectively fine-tunes only a very small portion of the model parameters, specifically only the Pre-LayerNorm parameters of the vision encoder within a VLM. Overcoming the existing tradeoff between performance and communication/computation efficiency in few-shot FL, NormFit sets a new benchmark by simultaneously achieving superior accuracy and substantially reduced communication and computational demands. Theoretically, we show that NormFit yields a considerably smaller generalization gap compared to tuning all LayerNorm parameters. Importantly, NormFit can function effectively as a standalone solution or integrate seamlessly with existing few-shot fine-tuning methods to further enhance their performance. Notably, NormFit offers implementation simplicity, achieving these improvements without any algorithmic modifications, changes to the underlying model architecture, or the addition of external parameters. 2

• Corresponding author 2 The code is available at https://github.com/AziMtmd/NormFit. 39th Conference on Neural Information Processing Systems (NeurIPS 2025). Method Trainable params (K) Comm. cost (KB) Comp. cost (GFLOPs) Full fine-tuning 1.5 × 10 5 4.1 × 10 5 8.4 × 10 5 Standard VLM few-shot methods adopted in FL CoOp 2.5 × 10 1 6.8 × 10 1 2.3 × 10 2 CoCoOp 8.9 × 10 2 2.4 × 10 3 2.9 × 10 5 TIP-Adapter-F 7.9 × 10 2 2.2 × 10 3 3.1 × 10 5 FL VLM few-shot methods FedCLIP 5.3 × 10 2 1.5 × 10 3 1.0 × 10 5