Instead of reduce function, try to use parentheses for better performance.

%dw 2.0

output application/json

var noOfRow= 1 to sizeOf(payload.rows default '')

var keys = payload.headers

var values = payload.rows

---

noOfRow map((item, index) -> {((keys map ((keyValue, keyIndex) ->

(keyValue) : values[index][keyIndex]

)))})