Fix quadratic rendering time for many inline links

Scanning from index 0 after every inline-pattern match rescanned the
unprocessed text repeatedly, making conversion quadratic in the number
of inline elements. Return the index just past the inserted placeholder
so the next scan starts at the unprocessed tail.

Fixes #1619.

Author: aryansk

docs/changelog.md

@@ -15,6 +15,7 @@ See the [Contributing Guide](contributing.md) for details.
 ### Fixed
 
 * Fix an issue with excessive backtracking when matching inline code blocks (#1617).
+* Fix quadratic rendering time when a paragraph contains many inline links (#1619).
 
 ## [3.10.3] - 2026-07-30
 

markdown/treeprocessors.py

@@ -323,12 +323,17 @@ def __applyPattern(
         placeholder = self.__stashNode(node, pattern.type())
 
         if new_style:
+            # Return the index just past the inserted placeholder so the
+            # next call scans only the unprocessed tail. Scanning from 0
+            # after every match makes repeated inline patterns quadratic.
             return "{}{}{}".format(data[:start],
-                                   placeholder, data[end:]), True, 0
+                                   placeholder, data[end:]), True, start + len(placeholder)
         else:  # pragma: no cover
             return "{}{}{}{}".format(leftData,
                                      match.group(1),
-                                     placeholder, match.groups()[-1]), True, 0
+                                     placeholder, match.groups()[-1]), True, (
+                len(leftData) + len(match.group(1)) + len(placeholder)
+            )
 
     def __build_ancestors(self, parent: etree.Element | None, parents: list[str]) -> None:
         """Build the ancestor list."""

tests/test_syntax/inline/test_links.py

@@ -21,6 +21,8 @@
 
 from markdown.test_tools import TestCase
 
+import time
+
 
 class TestInlineLinks(TestCase):
 
@@ -434,3 +436,21 @@ def test_ref_round_brackets(self):
                 """
             )
         )
+
+    def test_many_repeated_links(self):
+        # Regression test for #1619: rendering many inline links in one
+        # paragraph used to rescan the unprocessed text from the start after
+        # every match, making conversion quadratic in the number of links.
+        from markdown import Markdown
+
+        text = "[link](x)" * 8192
+        start = time.monotonic()
+        html = Markdown().convert(text)
+        elapsed = time.monotonic() - start
+
+        # The old implementation takes several seconds (or more on slow CI)
+        # for this input; the linear implementation finishes well under a
+        # second on any machine. Allow a generous ceiling to avoid flakes.
+        self.assertLess(elapsed, 5)
+        self.assertEqual(html.count("<a "), 8192)
+        self.assertEqual(html.count("</a>"), 8192)