diff --git a/src/copilot_usage/cli.py b/src/copilot_usage/cli.py
index 9a59e6a..10b62ac 100644
--- a/src/copilot_usage/cli.py
+++ b/src/copilot_usage/cli.py
@@ -330,9 +330,21 @@ def session(ctx: click.Context, session_id: str, path: Path | None) -> None:
             click.echo("No sessions found.", err=True)
             sys.exit(1)
 
-        # Parse all sessions and find the one matching by prefix
+        # Fast path: skip directories that clearly cannot match the prefix.
+        # Only apply the pre-filter on UUID-shaped directory names (36 chars
+        # with 4 dashes), where the directory name IS the session ID.
+        # Non-UUID dirs (e.g. test fixtures) always need a full parse.
         available: list[str] = []
         for events_path in event_paths:
+            dir_name = events_path.parent.name
+            is_uuid_dir = len(dir_name) == 36 and dir_name.count("-") == 4
+            if (
+                len(session_id) >= 4
+                and is_uuid_dir
+                and not dir_name.startswith(session_id)
+            ):
+                available.append(dir_name[:8])
+                continue
             events = parse_events(events_path)
             if not events:
                 continue
diff --git a/tests/copilot_usage/test_cli.py b/tests/copilot_usage/test_cli.py
index 604a935..20786f8 100644
--- a/tests/copilot_usage/test_cli.py
+++ b/tests/copilot_usage/test_cli.py
@@ -34,9 +34,10 @@ def _write_session(
     premium: int = 3,
     output_tokens: int = 1500,
     active: bool = False,
+    use_full_uuid_dir: bool = False,
 ) -> Path:
     """Create a minimal events.jsonl file inside *base*/<dir>/."""
-    session_dir = base / session_id[:8]
+    session_dir = base / (session_id if use_full_uuid_dir else session_id[:8])
     session_dir.mkdir(parents=True, exist_ok=True)
 
     events: list[dict[str, Any]] = [
@@ -899,3 +900,146 @@ def _fake_read(timeout: float = 0.5) -> str | None:  # noqa: ARG001
     assert result.exit_code == 0
     # _show_session_by_index called at least twice: initial '1' + auto-refresh
     assert len(show_detail_calls) >= 2
+
+
+# ---------------------------------------------------------------------------
+# Issue #138 — fast pre-filter on directory name
+# ---------------------------------------------------------------------------
+
+
+def test_session_prefilter_skips_non_matching_dirs(
+    tmp_path: Path, monkeypatch: Any
+) -> None:
+    """parse_events is only called for the matching directory when prefix ≥ 4 chars.
+
+    Creates ≥ 5 UUID-named sessions and verifies the pre-filter skips parsing
+    directories whose names don't start with the requested prefix.
+    """
+    uuids = [
+        "aaaaaaaa-1111-1111-1111-111111111111",
+        "bbbbbbbb-2222-2222-2222-222222222222",
+        "cccccccc-3333-3333-3333-333333333333",
+        "dddddddd-4444-4444-4444-444444444444",
+        "eeeeeeee-5555-5555-5555-555555555555",
+    ]
+    target = uuids[2]  # cccccccc-...
+
+    for uid in uuids:
+        _write_session(tmp_path, uid, use_full_uuid_dir=True)
+
+    def _fake_discover(_base: Path | None = None) -> list[Path]:
+        return sorted(
+            tmp_path.glob("*/events.jsonl"),
+            key=lambda p: p.stat().st_mtime,
+            reverse=True,
+        )
+
+    monkeypatch.setattr("copilot_usage.cli.discover_sessions", _fake_discover)
+
+    parse_calls: list[Path] = []
+    original_parse = __import__(
+        "copilot_usage.parser", fromlist=["parse_events"]
+    ).parse_events
+
+    def _tracking_parse(events_path: Path) -> list[Any]:
+        parse_calls.append(events_path)
+        return original_parse(events_path)
+
+    monkeypatch.setattr("copilot_usage.cli.parse_events", _tracking_parse)
+
+    runner = CliRunner()
+    result = runner.invoke(main, ["session", "cccccccc"])
+    assert result.exit_code == 0
+    assert "cccccccc" in result.output
+
+    # Only the matching directory should have been parsed
+    assert len(parse_calls) == 1
+    assert parse_calls[0].parent.name == target
+
+
+def test_session_prefilter_short_prefix_parses_all(
+    tmp_path: Path, monkeypatch: Any
+) -> None:
+    """Short prefixes (< 4 chars) bypass the pre-filter and parse all sessions."""
+    uuids = [
+        "ab111111-aaaa-aaaa-aaaa-aaaaaaaaaaaa",
+        "ab222222-bbbb-bbbb-bbbb-bbbbbbbbbbbb",
+        "cd333333-cccc-cccc-cccc-cccccccccccc",
+    ]
+    for uid in uuids:
+        _write_session(tmp_path, uid, use_full_uuid_dir=True)
+
+    def _fake_discover(_base: Path | None = None) -> list[Path]:
+        # Sort reverse-alphabetically so cd333… (non-matching) is visited
+        # before ab… dirs, proving the pre-filter didn't skip it.
+        return sorted(
+            tmp_path.glob("*/events.jsonl"),
+            key=lambda p: p.parent.name,
+            reverse=True,
+        )
+
+    monkeypatch.setattr("copilot_usage.cli.discover_sessions", _fake_discover)
+
+    parse_calls: list[Path] = []
+    original_parse = __import__(
+        "copilot_usage.parser", fromlist=["parse_events"]
+    ).parse_events
+
+    def _tracking_parse(events_path: Path) -> list[Any]:
+        parse_calls.append(events_path)
+        return original_parse(events_path)
+
+    monkeypatch.setattr("copilot_usage.cli.parse_events", _tracking_parse)
+
+    runner = CliRunner()
+    # "ab" is only 2 chars — pre-filter should NOT skip anything
+    result = runner.invoke(main, ["session", "ab"])
+    assert result.exit_code == 0
+
+    # The non-matching cd333… dir must have been parsed (no pre-filter applied).
+    assert len(parse_calls) >= 2
+    parsed_dirs = {p.parent.name for p in parse_calls}
+    assert "cd333333-cccc-cccc-cccc-cccccccccccc" in parsed_dirs
+
+
+def test_session_prefilter_non_uuid_dirs_always_parsed(
+    tmp_path: Path, monkeypatch: Any
+) -> None:
+    """Non-UUID directory names are always parsed even with long prefix."""
+    session_dir = tmp_path / "corrupt-session"
+    session_dir.mkdir()
+    events: list[dict[str, Any]] = [
+        {
+            "type": "session.start",
+            "timestamp": "2025-01-15T10:00:00Z",
+            "data": {
+                "sessionId": "corrupt0-0000-0000-0000-000000000000",
+                "startTime": "2025-01-15T10:00:00Z",
+                "context": {"cwd": "/home/user"},
+            },
+        },
+        {
+            "type": "session.shutdown",
+            "timestamp": "2025-01-15T11:00:00Z",
+            "currentModel": "claude-sonnet-4",
+            "data": {
+                "shutdownType": "normal",
+                "totalPremiumRequests": 1,
+                "totalApiDurationMs": 100,
+                "modelMetrics": {},
+            },
+        },
+    ]
+    with (session_dir / "events.jsonl").open("w") as fh:
+        for ev in events:
+            fh.write(json.dumps(ev) + "\n")
+
+    def _fake_discover(_base: Path | None = None) -> list[Path]:
+        return list(tmp_path.glob("*/events.jsonl"))
+
+    monkeypatch.setattr("copilot_usage.cli.discover_sessions", _fake_discover)
+
+    runner = CliRunner()
+    result = runner.invoke(main, ["session", "corrupt0"])
+    assert result.exit_code == 0
+    assert "corrupt0" in result.output