Most efficient way to save a file on disk while executing async work, on a ASP.Net CORE Web Request

On a request on my Web API, I’m saving a image to the disk and also processing it with an external API which usually takes several seconds. This is a high traffic API, thus I would like to design it in the most efficient way. The image comes in Base64 “encoding”, but this is not pertinent. We can think about it as an arbitrary byte array of 150KB in average (so the saving to disk operation should be really fast).

Can’t redirect to another action method after form submission

I want to redirect to an action that says Order is successful, but when the form is submitted, it redirects to the same action /Checkout with error: This page isn't working. This is understandble since the query strings are not present, but this doesn’t matter to me. I want
it to redirect to /OrderComplete instead. The debugger confirmed ModelState is valid and RedirectToAction is hit every time.