I've struggled debugging a slow put-request to our ASP.NET Core 2.0 Web API. After trying most things, I realized turning Fiddler off solved it. I've confirmed this, enabling the listener adds about 1s overhead to the request. This is only present for PUT requests.
POST-request with Fiddler on: ~20ms
Identical request with PUT, Fiddler on: ~1.200ms
Identical request with PUT, Fiddler off: ~20ms