fix?: reduce parallel visitors

Author: erikwrede

tests/benchmarks/test_visit.py

@@ -23,5 +23,5 @@ def test_visit_all_ast_nodes(benchmark, big_schema_sdl):  # noqa: F811
 def test_visit_all_ast_nodes_in_parallel(benchmark, big_schema_sdl):  # noqa: F811
     document_ast = parse(big_schema_sdl)
     visitor = DummyVisitor()
-    parallel_visitor = ParallelVisitor([visitor] * 50)
+    parallel_visitor = ParallelVisitor([visitor] * 20)
     benchmark(lambda: visit(document_ast, parallel_visitor))