The Prefill Wall: Why MTP's 2 Barely Moves Long-Context Latency (Qwen3.6-27B, RTX 3090)

My MTP post showed multi-token prediction roughly doubling Qwen3.6-27B's generation on a 3090. A reader asked the question I'd skipped: what about prompt processing at long context? So I measured it — and that turns out to be the real wall, the one MTP can't climb. On a single RTX 3090, prefill (pro