diff --git a/detect_secrets/plugins/private_key.py b/detect_secrets/plugins/private_key.py
index 9cec82857..744435ccd 100644
--- a/detect_secrets/plugins/private_key.py
+++ b/detect_secrets/plugins/private_key.py
@@ -68,6 +68,12 @@ class PrivateKeyDetector(RegexBasedDetector):
 
     def __init__(self) -> None:
         self._analyzed_files: Set[str] = set()
+        # Tracks files whose on-disk size has already been checked, regardless of
+        # whether that size fell within the scannable range. Without this, files
+        # outside the size range (e.g. larger than MAX_FILE_SIZE) would never be
+        # recorded in ``_analyzed_files``, causing ``get_file_size`` to re-run for
+        # every single line of the file -- a per-file operation executed per-line.
+        self._sized_files: Set[str] = set()
         self._commit_hashes: Set[Tuple[str, str]] = set()
 
     def analyze_line(
@@ -111,9 +117,16 @@ def analyze_line(
                 self._commit_hashes.add((filename, commit_hash))
             return output
 
-        if filename not in self._analyzed_files \
-                and 0 < self.get_file_size(filename) < PrivateKeyDetector.MAX_FILE_SIZE:
-            self._analyzed_files.add(filename)
+        # Determine the file size at most once per file. Files whose size falls
+        # outside the scannable range are still recorded (via ``_sized_files``) so
+        # that we never re-run this filesystem lookup on subsequent lines.
+        if filename not in self._sized_files:
+            self._sized_files.add(filename)
+            if 0 < self.get_file_size(filename) < PrivateKeyDetector.MAX_FILE_SIZE:
+                self._analyzed_files.add(filename)
+
+        if filename in self._analyzed_files:
+            self._analyzed_files.discard(filename)
             file_content = self.read_file(filename)
             if file_content:
                 found_secrets = super().analyze_line(
diff --git a/tests/plugins/private_key_test.py b/tests/plugins/private_key_test.py
index 198c57cfa..b3115d86e 100644
--- a/tests/plugins/private_key_test.py
+++ b/tests/plugins/private_key_test.py
@@ -1,4 +1,5 @@
 import json
+from unittest import mock
 
 import pytest
 
@@ -150,6 +151,65 @@ def test_private_key_line_number_2():
         )
 
 
+def test_get_file_size_is_called_at_most_once_per_file():
+    """Regression test for a performance bug.
+
+    ``PrivateKeyDetector.analyze_line`` runs once per line. It must not perform
+    a filesystem ``getsize`` lookup on every line: for a file larger than
+    ``MAX_FILE_SIZE`` (which is never added to the ``_analyzed_files`` cache),
+    the size lookup used to fire once per line, making the cost scale with
+    (files x lines) instead of (files). This test proves the file-size lookup
+    happens at most once for the whole file.
+    """
+    # Build a file that is well over MAX_FILE_SIZE (8 KiB) and has many lines,
+    # none of which contain a private key.
+    line = 'this is a perfectly ordinary line with no secrets in it at all'
+    file_content = '\n'.join(line for _ in range(500))
+    assert len(file_content.encode()) > (8 * 1024)
+
+    with mock_named_temporary_file() as f:
+        f.write(file_content.encode())
+        f.seek(0)
+
+        with mock.patch(
+            'detect_secrets.plugins.private_key.os.path.getsize',
+            wraps=__import__('os').path.getsize,
+        ) as mock_getsize:
+            secrets = SecretsCollection()
+            secrets.scan_file(f.name)
+
+        assert mock_getsize.call_count <= 1, (
+            f'Expected the private-key file-size lookup to run at most once per '
+            f'file, but it ran {mock_getsize.call_count} times.'
+        )
+
+
+def test_multiline_private_key_in_small_file_is_still_detected():
+    """Guards the file-size fix.
+
+    The size lookup was hoisted to run once per file, and the per-file content
+    read now happens exactly once. This confirms the whole-file read path still
+    fires for a small file, so a private key split across multiple lines (which
+    only matches when the full file content is scanned) is still detected.
+    """
+    file_content = '\n'.join([
+        'Irrelevant line',
+        '-----BEGIN RSA PRIVATE KEY-----',
+        'MIIBVwIBADANBgkqhkiG9w0BAQEFAASC',
+        '-----END RSA PRIVATE KEY-----',
+    ])
+    assert len(file_content.encode()) < (8 * 1024)
+
+    with mock_named_temporary_file() as f:
+        f.write(file_content.encode())
+        f.seek(0)
+
+        secrets = SecretsCollection()
+        secrets.scan_file(f.name)
+
+    assert len(list(secrets)) == 1
+
+
 @pytest.fixture(autouse=True)
 def configure_plugins():
     with transient_settings({