Root cause of ~1 t/s long-context decode: llama.cpp build has GGML_CUDA_FA_ALL_QUANTS=OFF, and FA requires K type == V type. Mixed cache q8_0/f16 silently disabled FA, collapsing decode at 200k ctx. Set V to q8_0 (halves V memory too) and retune threads 12/1 for 16 Zen5 cores.