Since prompt evaluation (prefill) is slower than generation (decoding), the bottleneck is likely related to **compute throughput** or **batch processing** rather than memory bandwidth.

### 1. GPU Offloading
*   **Check Layer Count:** Ensure all layers are actually on the GPU. If some layers are falling back to the CPU, prefill will be significantly slower.
    *   *Action:* Increase `-ngl` (or `--n-gpu-layers`) until all layers are offloaded.
*   **VRAM Overhead:** If the model + context window exceeds VRAM, it will swap to system RAM.
    *   *Action:* Check `nvidia-smi` during prefill to ensure VRAM isn't maxed out.

### 2. Batch Size (The most likely culprit)
*   **Increase Batch Size:** `llama.cpp` processes the prompt in batches. If the batch size is too small, it cannot utilize the GPU's parallel processing power.
    *   *Action:* Increase `-b` (or `--batch-size`) to `2048` or `4096`.
*   **Batch Timeout:** If using a server, ensure the request isn't being throttled by a small internal buffer.

### 3. Flash Attention
*   **Enable Flash Attention:** This is critical for prefill speed as it reduces the quadratic complexity of the attention mechanism.
    *   *Action:* Ensure `--flash-attn` is enabled in your startup command.

### 4. Context Window
*   **Context Size:** A massive context window (`-c` or `--ctx-size`) increases the memory footprint and computation time for every token in the prompt.
    *   *Action:* Reduce `-c` to the minimum required for your specific use case.

### 5. Quantization & Kernels
*   **Check Quant Type:** Some quantization types (e.g., certain bits) may not have optimized CUDA kernels, forcing slower paths.
    *   *Action:* Use standard `Q4_K_M` or `Q8_0` formats.
*   **Update llama.cpp:** Ensure you are using the latest build, as prefill optimizations are updated frequently.
    *   *Action:* Recompile with `LLAMA_CUDA=1` (or `LLAMA_ROCM=1`) to ensure latest kernels are used.

### Summary Checklist for Command Line:
1.  `--n-gpu-layers` $\rightarrow$ Maximize.
2.  `--batch-size` $\rightarrow$ Increase to `2048`.
3.  `--flash-attn` $\rightarrow$ Enable.
4.  `--ctx-size` $\rightarrow$ Right-size.