improve performance

proto/message.py

@@ -524,19 +524,20 @@ def __init__(
         # coerced.
         marshal = self._meta.marshal
         for key, value in mapping.items():
-            # Underscores may be appended to field names
-            # that collide with python or proto-plus keywords.
-            # In case a key only exists with a `_` suffix, coerce the key
-            # to include the `_` suffix. Is not possible to
-            # natively define the same field with a trailing underscore in protobuf.
-            # See related issue
-            # https://github.com/googleapis/python-api-core/issues/227
-            if key not in self._meta.fields and f"{key}_" in self._meta.fields:
-                key = f"{key}_"
-
             try:
                 pb_type = self._meta.fields[key].pb_type
             except KeyError:
+                # Underscores may be appended to field names
+                # that collide with python or proto-plus keywords.
+                # In case a key only exists with a `_` suffix, coerce the key
+                # to include the `_` suffix. Is not possible to
+                # natively define the same field with a trailing underscore in protobuf.
+                # See related issue
+                # https://github.com/googleapis/python-api-core/issues/227
+                if f"{key}_" in self._meta.fields:
+                    pb_type = self._meta.fields[f"{key}_"].pb_type
+                    continue
+
                 if ignore_unknown_fields:
                     continue