diff --git a/conda_build/metadata.py b/conda_build/metadata.py
index 7a0f9da1a7..ed55166733 100644
--- a/conda_build/metadata.py
+++ b/conda_build/metadata.py
@@ -36,7 +36,7 @@
 )
 
 if TYPE_CHECKING:
-    from typing import Any, Literal
+    from typing import Literal
 
 try:
     import yaml
@@ -110,6 +110,10 @@ def remove_constructor(cls, tag):
 # used to avoid recomputing/rescanning recipe contents for used variables
 used_vars_cache = {}
 
+# Placeholders singletons for os/os.environ used to memoize select_lines().
+_selector_placeholder_os = object()
+_selector_placeholder_os_environ = object()
+
 
 def get_selectors(config: Config) -> dict[str, bool]:
     """Aggregates selectors for use in recipe templating.
@@ -268,64 +272,92 @@ def eval_selector(selector_string, namespace, variants_in_place):
         return eval_selector(next_string, namespace, variants_in_place)
 
 
-@lru_cache(maxsize=None)
-def _split_line_selector(text: str) -> tuple[tuple[str | None, str], ...]:
-    lines: list[tuple[str | None, str]] = []
-    for line in text.splitlines():
-        line = line.rstrip()
+def select_lines(data, namespace, variants_in_place):
+    # Try to turn namespace into a hashable representation for memoization.
+    try:
+        namespace_copy = namespace.copy()
+        if namespace_copy.get("os") is os:
+            namespace_copy["os"] = _selector_placeholder_os
+        if namespace_copy.get("environ") is os.environ:
+            namespace_copy["environ"] = _selector_placeholder_os_environ
+        if "pin_run_as_build" in namespace_copy:
+            # This raises TypeError if pin_run_as_build is not a dict of dicts.
+            try:
+                namespace_copy["pin_run_as_build"] = tuple(
+                    (key, tuple((k, v) for k, v in value.items()))
+                    for key, value in namespace_copy["pin_run_as_build"].items()
+                )
+            except (AttributeError, TypeError, ValueError):
+                # AttributeError: no .items method
+                # TypeError: .items() not iterable of iterables
+                # ValueError: .items() not iterable of only (k, v) tuples
+                raise TypeError
+        for k, v in namespace_copy.items():
+            # Convert list/sets/tuple to tuples (of tuples if it contains
+            # list/set elements). Copy any other type verbatim and rather fall
+            # back to the non-memoized version to avoid wrong/lossy conversions.
+            if isinstance(v, (list, set, tuple)):
+                namespace_copy[k] = tuple(
+                    tuple(e) if isinstance(e, (list, set)) else e
+                    for e in v
+                )
+        namespace_tuple = tuple(namespace_copy.items())
+        # Raise TypeError if anything in namespace_tuple is not hashable.
+        hash(namespace_tuple)
+    except TypeError:
+        return _select_lines(data, namespace, variants_in_place)
+    return _select_lines_memoized(data, namespace_tuple, variants_in_place)
 
-        # skip comment lines, include a blank line as a placeholder
-        if line.lstrip().startswith("#"):
-            lines.append((None, ""))
-            continue
 
-        # include blank lines
-        if not line:
-            lines.append((None, ""))
-            continue
+@lru_cache(maxsize=200)
+def _select_lines_memoized(data, namespace_tuple, variants_in_place):
+    # Convert namespace_tuple to dict and undo the os/environ/pin_run_as_build
+    # replacements done in select_lines.
+    namespace = dict(namespace_tuple)
+    if namespace.get("os") is _selector_placeholder_os:
+        namespace["os"] = os
+    if namespace.get("environ") is _selector_placeholder_os_environ:
+        namespace["environ"] = os.environ
+    if "pin_run_as_build" in namespace:
+        namespace["pin_run_as_build"] = {
+            key: dict(value)
+            for key, value in namespace["pin_run_as_build"]
+        }
+    return _select_lines(data, namespace, variants_in_place)
+
+
+def _select_lines(data, namespace, variants_in_place):
+    lines = []
+
+    for i, line in enumerate(data.splitlines()):
+        line = line.rstrip()
 
-        # user may have quoted entire line to make YAML happy
         trailing_quote = ""
         if line and line[-1] in ("'", '"'):
             trailing_quote = line[-1]
 
-        match = sel_pat.match(line)
-        if match and (selector := match.group(3)):
-            # found a selector
-            lines.append((selector, (match.group(1) + trailing_quote).rstrip()))
+        if line.lstrip().startswith("#"):
+            # Don't bother with comment only lines
+            continue
+        m = sel_pat.match(line)
+        if m:
+            cond = m.group(3)
+            try:
+                if eval_selector(cond, namespace, variants_in_place):
+                    lines.append(m.group(1) + trailing_quote)
+            except Exception as e:
+                sys.exit(
+                    """\
+Error: Invalid selector in meta.yaml line %d:
+offending line:
+%s
+exception:
+%s
+"""
+                    % (i + 1, line, str(e))
+                )
         else:
-            # no selector found
-            lines.append((None, line))
-    return tuple(lines)
-
-
-def select_lines(text: str, namespace: dict[str, Any], variants_in_place: bool) -> str:
-    lines = []
-    selector_cache: dict[str, bool] = {}
-    for i, (selector, line) in enumerate(_split_line_selector(text)):
-        if not selector:
-            # no selector? include line as is
             lines.append(line)
-        else:
-            # include lines with a selector that evaluates to True
-            try:
-                if selector_cache[selector]:
-                    lines.append(line)
-            except KeyError:
-                # KeyError: cache miss
-                try:
-                    value = bool(eval_selector(selector, namespace, variants_in_place))
-                    selector_cache[selector] = value
-                    if value:
-                        lines.append(line)
-                except Exception as e:
-                    sys.exit(
-                        f"Error: Invalid selector in meta.yaml line {i + 1}:\n"
-                        f"offending line:\n"
-                        f"{line}\n"
-                        f"exception:\n"
-                        f"{e.__class__.__name__}: {e}\n"
-                    )
     return "\n".join(lines) + "\n"