diff --git a/collectionparser/api.py b/collectionparser/api.py
index ce7e726..52e23f4 100644
--- a/collectionparser/api.py
+++ b/collectionparser/api.py
@@ -11,8 +11,10 @@ from tqdm import tqdm
 from collectionparser.utils import create_dir_for_file, find_files
 
 # ponytail: buckets under this size sort byte-exactly in RAM; larger buckets
-# shell out to `sort -u`, which spills to disk.
-_IN_MEMORY_SORT_LIMIT = 8 * 1024 * 1024
+# shell out to `sort -u`. 64 MB covers almost every bucket, so the final dedup
+# rarely spawns a process; a single bucket's sort is one file, so memory stays
+# bounded. Lower this if bucket files can be huge.
+_IN_MEMORY_SORT_LIMIT = 64 * 1024 * 1024
 
 
 class API:
@@ -80,9 +82,13 @@ def sort_unique(src, dst) -> None:
         Path(dst).write_bytes(b"\n".join(lines) + b"\n" if lines else b"")
     else:
         tmp = f"{dst}.tmp"
+        # Parallel sort, but memory-bounded: `-S` caps the sort buffer and
+        # `--parallel 4` bounds the threads. A large `-S` here can OOM and get
+        # the sort SIGKILLed. ponytail: ceiling ~1 GB + per-thread buffers;
+        # raise `-S`/`--parallel` only if the box has RAM to spare.
         with open(tmp, "wb") as writer:
             subprocess.run(
-                ["sort", "-u", str(src)],
+                ["sort", "-S", "1G", "--parallel", "4", "-u", str(src)],
                 stdout=writer,
                 check=True,
                 env={**os.environ, "LC_ALL": "C"},
diff --git a/collectionparser/build.py b/collectionparser/build.py
index f3de4da..5dc8a14 100644
--- a/collectionparser/build.py
+++ b/collectionparser/build.py
@@ -246,8 +246,14 @@ def build(
     dataset.mkdir(parents=True, exist_ok=True)
     scratch.mkdir(parents=True, exist_ok=True)
     os.environ["TMPDIR"] = str(scratch)  # `sort -u` spills here, not into the dataset
-    for stale in scratch.glob("extract_*"):
-        shutil.rmtree(stale, ignore_errors=True)
+    # Leftovers from a crashed run: extraction dirs, and `sort` run/temp files.
+    # Only scratch is touched, never the source or dataset.
+    for pattern in ("extract_*", "sort*", "*.tmp"):
+        for stale in scratch.glob(pattern):
+            if stale.is_dir():
+                shutil.rmtree(stale, ignore_errors=True)
+            else:
+                stale.unlink(missing_ok=True)
 
     if extract and not _which("7z", "7za", "unrar"):
         print(
@@ -316,17 +322,27 @@ def build(
         pending = [f for f in inputs if str(f) not in parsed]
 
         t0 = time.time()
+        pairs: int | None = None
+        rejects: int | None = None
         if pending:
-            parse_files(
+            pairs, rejects = parse_files(
                 pending,
                 str(success),
                 str(failure),
                 progress_filename=str(progress_path),
                 hashes_filename=str(hashes),
             )
+            # Remember the counts so a later resume skips the full re-read.
+            state.setdefault("counts", {})[str(index)] = [pairs, rejects]
+            save()
         if success.exists():
-            pairs = _count_lines(success)
-            rejects = _count_lines(failure) if failure.exists() else 0
+            if pairs is None:
+                saved = (state.get("counts") or {}).get(str(index))
+                if saved:
+                    pairs, rejects = saved
+                else:  # ponytail: only pre-count-state runs need this full read
+                    pairs = _count_lines(success)
+                    rejects = _count_lines(failure) if failure.exists() else 0
             sort_unique(success, success)
             split_file(str(success), str(dataset))
             success.unlink(missing_ok=True)