perf: move resolve classes to validate

Author: maartenbreddels

traitlets/traitlets.py

@@ -2134,6 +2134,12 @@ class or its subclasses.  Our implementation is quite different
         super().__init__(**kwargs)
 
     def validate(self, obj, value):
+        # We can't do resolve the classes in subclass_init because that
+        # might be called before all imports are done.
+        # but instance_init will give us runtime overhead for every trait
+        # (even those that are not set / None).
+        # This place is a good compromise.
+        self._resolve_classes()
         assert self.klass is not None
         if isinstance(value, self.klass):  # type:ignore[arg-type]
             return value
@@ -2149,10 +2155,8 @@ def info(self):
             result += " or None"
         return result
 
-    def instance_init(self, obj):
-        # we can't do this in subclass_init because that
-        # might be called before all imports are done.
-        self._resolve_classes()
+    def subclass_init(self, cls):
+        pass
 
     def _resolve_classes(self):
         if isinstance(self.klass, str):