Add benchmark to Flash README (#376)

Author: rejojer

pageindex/flash/README.md

@@ -38,6 +38,37 @@ missing, non-PDF, encrypted, empty, or unreadable file.
 
 Page indexes are 1-based. `nodes` nests the same shape recursively.
 
+## Benchmark
+
+Nine PDFs, each run end to end with tree optimization: PDF parse, layout
+outline, merge, LLM expand, then a summary for every node.
+
+![Time against document length](assets/time_vs_pages.png)
+
+![Cost against document length](assets/cost_vs_pages.png)
+
+Both scale close to linearly with length, at 218 s and $0.85 per 1,000 pages.
+Two qualifiers. Cost follows node count a little more closely than page count,
+$0.0007 to $0.0016 per node, so a densely structured document costs more than
+its length suggests. And wall clock flattens past roughly 700 pages, where
+summary concurrency rather than length becomes the limit.
+
+| Document | Pages | Input tokens | Output tokens |
+|---|---:|---:|---:|
+| Bitcoin whitepaper | 9 | 8,715 | 4,673 |
+| Attention Is All You Need | 15 | 26,805 | 10,183 |
+| KIMI K3 | 47 | 85,704 | 35,217 |
+| DeepSeek-R1 | 86 | 68,398 | 26,351 |
+| Situational Awareness | 165 | 115,130 | 54,347 |
+| Federal Reserve 2023 report | 222 | 280,975 | 136,982 |
+| 9/11 Commission Report | 585 | 720,624 | 200,202 |
+| Pattern Recognition and Machine Learning | 758 | 857,983 | 277,675 |
+| Machine Learning: A Probabilistic Perspective | 1,098 | 1,587,265 | 646,958 |
+| **Total** | **2,985** | **3,751,599** | **1,392,588** |
+
+Measured with `gpt-5.6-luna` at $0.20 / $1.20 per million input / output tokens,
+priced cold with no prompt-cache discount.
+
 ## Limits
 
 - Scanned PDFs without embedded text are not supported.