But what if you've a complex app which interacts with GPIOs? You don't want to write the whole app a LMK and run in in kernel space, do you?

I've written a JSON-RPC API which can be used to interact with GPIOs. Currently I set/get a GPIO using sysfs. During handling of a request the operations on sysfs take most of the response time, far more time than parsing the request and writing the result to the client. How could I speed up the GPIO interaction with a LKM?