diff --git a/markdown_it/renderer.py b/markdown_it/renderer.py
index 37f6a1d1..6ccdee9b 100644
--- a/markdown_it/renderer.py
+++ b/markdown_it/renderer.py
@@ -85,18 +85,22 @@ def render(
         :param env: additional data from parsed input
 
         """
-        result = ""
+        # Collect the fragments and join them once: repeated ``str +=`` is only
+        # linear on interpreters that can extend a string in place (the default
+        # CPython build), and quadratic in the output size elsewhere (e.g. PyPy,
+        # GraalPy and free-threaded CPython 3.13).
+        out: list[str] = []
 
         for i, token in enumerate(tokens):
             if token.type == "inline":
                 if token.children:
-                    result += self.renderInline(token.children, options, env)
+                    out.append(self.renderInline(token.children, options, env))
             elif token.type in self.rules:
-                result += self.rules[token.type](tokens, i, options, env)
+                out.append(self.rules[token.type](tokens, i, options, env))
             else:
-                result += self.renderToken(tokens, i, options, env)
+                out.append(self.renderToken(tokens, i, options, env))
 
-        return result
+        return "".join(out)
 
     def renderInline(
         self, tokens: Sequence[Token], options: OptionsDict, env: EnvType
@@ -107,15 +111,15 @@ def renderInline(
         :param options: params of parser instance
         :param env: additional data from parsed input (references, for example)
         """
-        result = ""
+        out: list[str] = []
 
         for i, token in enumerate(tokens):
             if token.type in self.rules:
-                result += self.rules[token.type](tokens, i, options, env)
+                out.append(self.rules[token.type](tokens, i, options, env))
             else:
-                result += self.renderToken(tokens, i, options, env)
+                out.append(self.renderToken(tokens, i, options, env))
 
-        return result
+        return "".join(out)
 
     def renderToken(
         self,
@@ -205,20 +209,20 @@ def renderInlineAsText(
         :param options: params of parser instance
         :param env: additional data from parsed input
         """
-        result = ""
+        out: list[str] = []
 
         for token in tokens or []:
             if token.type in ("text", "code_inline"):
-                result += token.content
+                out.append(token.content)
             elif token.type == "image":
                 if token.children:
-                    result += self.renderInlineAsText(token.children, options, env)
+                    out.append(self.renderInlineAsText(token.children, options, env))
             elif token.type in ("html_inline", "html_block"):
-                result += token.content
+                out.append(token.content)
             elif token.type in ("softbreak", "hardbreak"):
-                result += "\n"
+                out.append("\n")
 
-        return result
+        return "".join(out)
 
     ###################################################
 
diff --git a/tests/test_api/test_renderer_results.py b/tests/test_api/test_renderer_results.py
new file mode 100644
index 00000000..d19d627e
--- /dev/null
+++ b/tests/test_api/test_renderer_results.py
@@ -0,0 +1,58 @@
+"""How the renderer combines the strings returned by its rules."""
+
+from types import MethodType
+
+import pytest
+
+from markdown_it import MarkdownIt
+from markdown_it.renderer import RendererHTML
+from markdown_it.token import Token
+
+
+def _renderer_with(values):
+    """A renderer whose ``custom`` rule returns ``values[idx]`` for token ``idx``."""
+    renderer = RendererHTML()
+    renderer.rules["custom"] = MethodType(
+        lambda self, tokens, idx, opts, env: values[idx], renderer
+    )
+    return renderer, [Token("custom", "", 0) for _ in values]
+
+
+class Wrapped(str):
+    """A ``str`` subclass whose overloaded addition must not be used."""
+
+    def __radd__(self, other):
+        raise AssertionError("not called")
+
+    __add__ = __radd__
+
+
+@pytest.mark.parametrize("method", ["render", "renderInline"])
+def test_render_rule_results_are_joined_into_a_plain_str(method):
+    # The fragments are joined in order, and the result is a plain ``str``
+    # even when a rule returns a ``str`` subclass (overloaded addition is not used).
+    renderer, tokens = _renderer_with(["a", Wrapped("b"), "c"])
+    result = getattr(renderer, method)(tokens, MarkdownIt().options, {})
+    assert type(result) is str
+    assert result == "abc"
+
+
+@pytest.mark.parametrize("method", ["render", "renderInline"])
+def test_render_rule_invalid_result_raises(method):
+    renderer, tokens = _renderer_with(["first", None, "third"])
+    with pytest.raises(TypeError):
+        getattr(renderer, method)(tokens, MarkdownIt().options, {})
+
+
+def test_render_inline_as_text_is_joined_into_a_plain_str():
+    # Image ``alt`` text is collected the same way: joined in order into a plain
+    # ``str``, without using a ``str`` subclass's overloaded addition.
+    def text(content):
+        token = Token("text", "", 0)
+        token.content = content
+        return token
+
+    tokens = [text("y"), Token("softbreak", "br", 0), text(Wrapped("x"))]
+    result = RendererHTML().renderInlineAsText(tokens, MarkdownIt().options, {})
+    assert type(result) is str
+    assert result == "y\nx"