Don't uselessly recombine registries each time we recurse into subsch…

…emas.

Also add a benchmark via @robherring to ensure we compare these timings
(validating with or without a registry which isn't really being used).

Fixes a ~4-5x slowdown. These are now on par with each other.

Closes: #1088

jsonschema/benchmarks/unused_registry.py

@@ -0,0 +1,35 @@
+"""
+An unused schema registry should not cause slower validation.
+
+"Unused" here means one where no reference resolution is occurring anyhow.
+
+See https://github.com/python-jsonschema/jsonschema/issues/1088.
+"""
+from pyperf import Runner
+from referencing import Registry
+from referencing.jsonschema import DRAFT201909
+
+from jsonschema import Draft201909Validator
+
+registry = Registry().with_resource(
+    "urn:example:foo",
+    DRAFT201909.create_resource({})
+)
+
+schema = {"$ref": "https://json-schema.org/draft/2019-09/schema"}
+instance = {"maxLength": 4}
+
+no_registry = Draft201909Validator(schema)
+with_useless_registry = Draft201909Validator(schema, registry=registry)
+
+if __name__ == "__main__":
+    runner = Runner()
+
+    runner.bench_func(
+        "no registry",
+        lambda: no_registry.is_valid(instance),
+    )
+    runner.bench_func(
+        "useless registry",
+        lambda: with_useless_registry.is_valid(instance),
+    )

jsonschema/validators.py

@@ -224,12 +224,7 @@ class Validator:
         format_checker: _format.FormatChecker | None = field(default=None)
         # TODO: include new meta-schemas added at runtime
         _registry: referencing.jsonschema.SchemaRegistry = field(
-            default=_DEFAULT_REGISTRY,
-            converter=lambda value: (
-                _DEFAULT_REGISTRY
-                if value is _DEFAULT_REGISTRY
-                else SPECIFICATIONS.combine(value)
-            ),
+            default=referencing.Registry(),
             kw_only=True,
             repr=False,
         )
@@ -274,9 +269,9 @@ def evolve(self, **changes):
 
         def __attrs_post_init__(self):
             if self._resolver is None:
-                self._resolver = self._registry.resolver_with_root(
-                    resource=specification.create_resource(self.schema),
-                )
+                registry = _DEFAULT_REGISTRY.combine(self._registry)
+                resource = specification.create_resource(self.schema)
+                self._resolver = registry.resolver_with_root(resource)
 
         @classmethod
         def check_schema(cls, schema, format_checker=_UNSET):