diff --git a/README.md b/README.md
index 124234e..fdda454 100644
--- a/README.md
+++ b/README.md
@@ -175,6 +175,45 @@ Point `ignoreFile` at e.g. `<repo>/slopo.ignore.txt` if you would rather those
 decisions were versioned with the code — they are a judgement about *your*
 codebase, and arguably belong in review.
 
+## What gets indexed
+
+Slopo does **not** read `.gitignore` — it walks the tree you point it at. Pointed
+at a repository root with no exclusions, it will happily parse every file under
+`node_modules`. That is not a review of your code; it is a review of your
+dependencies, and it is slow enough to matter.
+
+Measured on one real repository:
+
+| scope | files | code units | embedding at the reference rate |
+| --- | --- | --- | --- |
+| nothing excluded | ~1,300 | **113,768** | **~7.9 hours** |
+| default excludes | 562 | 8,607 | ~36 minutes |
+| default excludes + a server-only scope | **165** | **1,519** | **~6 minutes** |
+
+So there are two independent safeguards.
+
+**Default excludes**, on unless you turn them off: `node_modules`, `.git`,
+`dist`, `build`, `out`, `coverage`, `.venv`, `venv`, `__pycache__`, `target`,
+`vendor`, and minified bundles. Your own `sourceDirExclude` is **added** to
+these rather than replacing them — deliberately, because a settings patch
+replaces a whole value, so setting one pattern would otherwise silently
+re-enable the 13x blow-up.
+
+**A budget on the backlog.** `maxIndexUnits` (default 5,000) refuses to start an
+embed that exceeds it, naming the count, the estimate and the lever:
+
+> 113,768 code units still need embedding across 1,300 files. At the reference
+> rate that is about 7.9 hours, against a budget of 5,000.
+
+Nothing has been embedded at that point. Raise the budget, or narrow the scope —
+setting `maxIndexUnits` to 0 removes the limit. The count comes from reading
+slopo's own database, not from parsing its output, because `Indexed N code units`
+reports only what was *newly* indexed: on a warm database it reads zero while the
+backlog is still full, which is exactly the state a refusal leaves behind.
+
+Every action indexes first so the gate runs before anything expensive, and
+`status` reports both the effective excludes and the budget.
+
 ## Things worth knowing
 
 **Five settings are frozen at first index.** `source_dir`, `embedding_model`,