diff --git a/lib/datalog/api.sx b/lib/datalog/api.sx
index 01abec95..a76d3624 100644
--- a/lib/datalog/api.sx
+++ b/lib/datalog/api.sx
@@ -112,7 +112,8 @@
           (let
             ((existing (get (get db :facts) rel-key))
               (kept (list))
-              (kept-keys {}))
+              (kept-keys {})
+              (kept-index {}))
             (do
               (for-each
                 (fn
@@ -121,10 +122,19 @@
                     (not (dl-tuple-equal? t lit))
                     (do
                       (append! kept t)
-                      (dict-set! kept-keys (dl-tuple-key t) true))))
+                      (dict-set! kept-keys (dl-tuple-key t) true)
+                      (when
+                        (>= (len t) 2)
+                        (let ((k (dl-arg-key (nth t 1))))
+                          (do
+                            (when
+                              (not (has-key? kept-index k))
+                              (dict-set! kept-index k (list)))
+                            (append! (get kept-index k) t)))))))
                 existing)
               (dict-set! (get db :facts) rel-key kept)
-              (dict-set! (get db :facts-keys) rel-key kept-keys))))
+              (dict-set! (get db :facts-keys) rel-key kept-keys)
+              (dict-set! (get db :facts-index) rel-key kept-index))))
         ;; Wipe all relations that have a rule (these are IDB) so the
         ;; saturator regenerates them from the surviving EDB.
         (let ((rule-heads (dl-rule-head-rels db)))
@@ -133,7 +143,8 @@
               (k)
               (do
                 (dict-set! (get db :facts) k (list))
-                (dict-set! (get db :facts-keys) k {})))
+                (dict-set! (get db :facts-keys) k {})
+                (dict-set! (get db :facts-index) k {})))
             rule-heads))
         (dl-saturate! db)
         db))))
diff --git a/lib/datalog/db.sx b/lib/datalog/db.sx
index b5aea9b8..09f92315 100644
--- a/lib/datalog/db.sx
+++ b/lib/datalog/db.sx
@@ -12,7 +12,7 @@
 ;; lib/datalog/builtins.sx) swaps in the real `dl-rule-check-safety`,
 ;; which is order-aware and understands built-in predicates.
 
-(define dl-make-db (fn () {:facts {} :facts-keys {} :rules (list)}))
+(define dl-make-db (fn () {:facts {} :facts-keys {} :facts-index {} :rules (list)}))
 
 (define
   dl-rel-name
@@ -98,7 +98,8 @@
     (db rel-key)
     (let
       ((facts (get db :facts))
-        (fk (get db :facts-keys)))
+        (fk (get db :facts-keys))
+        (fi (get db :facts-index)))
       (do
         (when
           (not (has-key? facts rel-key))
@@ -106,8 +107,51 @@
         (when
           (not (has-key? fk rel-key))
           (dict-set! fk rel-key {}))
+        (when
+          (not (has-key? fi rel-key))
+          (dict-set! fi rel-key {}))
         (get facts rel-key)))))
 
+;; First-arg index helpers. Tuples are keyed by their first-after-rel
+;; arg's `(str ...)`; when that arg is a constant, dl-match-positive
+;; uses the index instead of scanning the full relation.
+(define
+  dl-arg-key
+  (fn
+    (v)
+    (str v)))
+
+(define
+  dl-index-add!
+  (fn
+    (db rel-key lit)
+    (let
+      ((idx (get db :facts-index))
+        (n (len lit)))
+      (when
+        (and (>= n 2) (has-key? idx rel-key))
+        (let
+          ((rel-idx (get idx rel-key))
+            (k (dl-arg-key (nth lit 1))))
+          (do
+            (when
+              (not (has-key? rel-idx k))
+              (dict-set! rel-idx k (list)))
+            (append! (get rel-idx k) lit)))))))
+
+(define
+  dl-index-lookup
+  (fn
+    (db rel-key arg-val)
+    (let
+      ((idx (get db :facts-index)))
+      (cond
+        ((not (has-key? idx rel-key)) (list))
+        (else
+          (let ((rel-idx (get idx rel-key))
+                (k (dl-arg-key arg-val)))
+            (if (has-key? rel-idx k) (get rel-idx k) (list))))))))
+
 (define dl-tuple-key (fn (lit) (str lit)))
 
 (define
@@ -140,6 +184,7 @@
                 (do
                   (dict-set! key-dict tk true)
                   (append! tuples lit)
+                  (dl-index-add! db rel-key lit)
                   true)))))))))
 
 ;; The full safety check lives in builtins.sx (it has to know which
diff --git a/lib/datalog/eval.sx b/lib/datalog/eval.sx
index 476fd0d0..b8ccd83f 100644
--- a/lib/datalog/eval.sx
+++ b/lib/datalog/eval.sx
@@ -24,7 +24,17 @@
         ((nil? rel) (error (str "dl-match-positive: bad literal " lit)))
         (else
           (let
-            ((tuples (dl-rel-tuples db rel)))
+            ;; If the first argument walks to a non-variable (constant
+            ;; or already-bound var), use the first-arg index for
+            ;; this relation. Otherwise scan the full tuple list.
+            ((tuples
+               (cond
+                 ((>= (len lit) 2)
+                   (let ((walked (dl-walk (nth lit 1) subst)))
+                     (cond
+                       ((dl-var? walked) (dl-rel-tuples db rel))
+                       (else (dl-index-lookup db rel walked)))))
+                 (else (dl-rel-tuples db rel)))))
             (do
               (for-each
                 (fn
diff --git a/lib/datalog/scoreboard.json b/lib/datalog/scoreboard.json
index 2545002b..3bc3e1e5 100644
--- a/lib/datalog/scoreboard.json
+++ b/lib/datalog/scoreboard.json
@@ -15,5 +15,5 @@
     {"name":"api","passed":11,"failed":0,"total":11},
     {"name":"demo","passed":15,"failed":0,"total":15}
   ],
-  "generated": "2026-05-08T09:20:09+00:00"
+  "generated": "2026-05-08T09:27:29+00:00"
 }
diff --git a/lib/datalog/tests/semi_naive.sx b/lib/datalog/tests/semi_naive.sx
index 914dcb2b..6df2a9e5 100644
--- a/lib/datalog/tests/semi_naive.sx
+++ b/lib/datalog/tests/semi_naive.sx
@@ -120,18 +120,20 @@
         (dl-sn-counts-agree?
           (dl-sn-counts "p(a). p(b). q(X) :- p(X), =(X, a)."))
         true)
-      ;; Chain length 10 — exercises multiple semi-naive iterations
-      ;; against the recursive ancestor rule.
+      ;; Chain length 12 — multiple semi-naive iterations against
+      ;; the recursive ancestor rule (differential vs naive).
       (dl-sn-test!
-        "chain-10 ancestor counts match"
-        (dl-sn-counts-agree? (dl-sn-counts (dl-sn-chain-source 10)))
+        "chain-12 ancestor counts match"
+        (dl-sn-counts-agree? (dl-sn-counts (dl-sn-chain-source 12)))
         true)
+      ;; Chain length 25 — semi-naive only — first-arg index makes
+      ;; this tractable in conformance budget.
       (dl-sn-test!
-        "chain-15 ancestor count value (semi only)"
+        "chain-25 ancestor count value (semi only)"
         (let
-          ((db (dl-program (dl-sn-chain-source 15))))
+          ((db (dl-program (dl-sn-chain-source 25))))
           (do (dl-saturate! db) (len (dl-relation db "ancestor"))))
-        120)
+        325)
       (dl-sn-test!
         "query through semi saturate"
         (let
diff --git a/plans/datalog-on-sx.md b/plans/datalog-on-sx.md
index f0233eec..c92f6e78 100644
--- a/plans/datalog-on-sx.md
+++ b/plans/datalog-on-sx.md
@@ -264,19 +264,33 @@ large graphs.
 
 ## Blockers
 
-- **Saturation perf** improving but not free. Resolved hash-set
-  membership in `dl-add-fact!` and replaced recursive `(rest lits)` in
-  `dl-find-bindings` with indexed iteration. chain-15 drops from ~25s
-  to ~16s and chain-25 saturates in ~33s real / 11s user — most CPU
-  now in unification (assoc-based subst dict copies) and dict
-  lookups during walks. Future: a per-rule "compiled" body that
-  pre-resolves arg positions and intern variable indices, then
-  unification can use array slots instead of dict assoc.
+- **Saturation perf**: three rounds done.
+  - hash-set membership in `dl-add-fact!` (Phase 5b)
+  - indexed iteration in `dl-find-bindings` (Phase 5c)
+  - first-arg index per relation (Phase 5e) — when a body literal's
+    first arg walks to a non-variable, dl-match-positive looks up
+    by `(str arg)` instead of scanning the full relation.
+  chain-25 saturation drops from ~33s to ~18s real (10s user).
+  chain-50 still long (~120s+) due to dict-copy overhead in
+  unification subst threading. Future: per-rule "compiled" body
+  with pre-resolved var positions, slot-based subst representation
+  to avoid `assoc` per binding.
 
 ## Progress log
 
 _Newest first._
 
+- 2026-05-08 — Phase 5e perf: first-arg index per relation. db gains
+  `:facts-index {<rel>: {<first-arg-key>: tuples}}` mirroring the
+  existing `:facts-keys` membership index. `dl-add-fact!` populates
+  it; `dl-match-positive` walks the body literal's first arg under
+  the current subst — if it's bound to a non-var, look up by
+  `(str arg)` and iterate only the matching subset. chain-25
+  saturation 33s → 18s real (~2x). chain-50 still slow (~120s+)
+  but tractable; next bottleneck is subst dict copies during
+  unification. Differential test bumped to chain-12, semi-only
+  count to chain-25.
+
 - 2026-05-08 — Demo: tag co-occurrence. `(cotagged P T1 T2)` — post
   has both T1 and T2 with T1 != T2 — and `(tag-pair-count T1 T2 N)`
   counting posts per distinct tag pair. Demonstrates count