Better reflect real-world API with bigger payloads

Hello,

First, congrats for these nice & useful benchmarks !

I made a very simple benchmark [here](https://github.com/abenhamdine/frameworks-benchmark) with fastify vs Express and found some intereresting data : see https://github.com/fastify/fastify/issues/178 for the background.

The important point is : **size of payload matters** : with a big JSON payload, json stringification becomes the bottleneck, and fastify matches Express performance only if a schema is applied in fastify route, see https://github.com/fastify/fastify/issues/178#issuecomment-328036894

I would be glad to send a PR to add a new route in the tests, in order to test
a bigger payload, or you can directly copy the test data I used in my benchmark if you prefer !