Skip the user_config deep copy unless esphome config --no-defaults needs it

tests/unit_tests/test_substitutions.py

@@ -370,7 +370,7 @@ def test_validate_config_captures_user_config_snapshot(tmp_path: Path) -> None:
     """
     test_config = _get_test_minimal_valid_config(tmp_path)
 
-    result = config_module.validate_config(test_config, None)
+    result = config_module.validate_config(test_config, None, snapshot_user_config=True)
 
     # Snapshot is populated.
     assert result.user_config is not None
@@ -393,7 +393,7 @@ def test_validate_config_user_config_snapshot_is_deep_copy(tmp_path: Path) -> No
     """
     test_config = _get_test_minimal_valid_config(tmp_path)
 
-    result = config_module.validate_config(test_config, None)
+    result = config_module.validate_config(test_config, None, snapshot_user_config=True)
 
     assert result.user_config is not None
     # preload_core_config injected build_path onto the validated config.
@@ -404,6 +404,20 @@ def test_validate_config_user_config_snapshot_is_deep_copy(tmp_path: Path) -> No
     assert result["esphome"] is not result.user_config["esphome"]
 
 
+def test_validate_config_skips_user_config_snapshot_by_default(
+    tmp_path: Path,
+) -> None:
+    """Without ``snapshot_user_config`` the deep copy is skipped entirely;
+    only ``esphome config --no-defaults`` needs the snapshot and the copy is
+    too expensive to take on every load.
+    """
+    test_config = _get_test_minimal_valid_config(tmp_path)
+
+    result = config_module.validate_config(test_config, None)
+
+    assert result.user_config is None
+
+
 def test_merge_config_preserves_ordered_dict() -> None:
     """Test that merge_config preserves OrderedDict type.