Concurrent Multipart requests

I need to handle large number of concurrent requests on my phoenix app each of type multipart. Each request can have 1 or 2 files and upload these files to a storage and return the response to client.
When I did a performance test on my app after a couple of seconds the requests started taking more than 10 seconds to complete even with small file size . I was looking for some insights on how could I tune my app to handle concurrent requests

As usual with any performance question we need information on:

  1. How are you testing this? what benchmark tool?
  2. How are you running your application?
  3. Can you show any code for your controller?