I wouldn't recommend this solution, because you didn't check the content and it may clog up your memory, but the fastest way to respond with a 200 OK is like this: https://modelshare.mendix.com/models/b3532aad-4103-48ae-89ce-5788e83224f9/post200asap
Do the preprocessing in a process queue?
Why would you return 200 before you know that the message has been received?
Do the minimum amount of work to assure that the message has arrived and is safely in the queue, and then return 200. If you don't, you might run into an exception when you have already returned 200, and you've lost the data.