When the reader's and writer's schemas are records and the reader's schema has a field with default value and the writer's schema doesn't have the field, the ResolvingDecoder keeps the default value in a byte array. This byte array is in Json format. Moving this to Avro binary format improves performance.
Apply the test patch and try "Perf -M". Then apply the patch and run it again. On my machine, the performance is three times the original.