httpErrors with EpiServer 9 not working in Windows 2012 R2 but are on Windows 10

I have an EPiServer web application that I am running which has a CMS controlled 404 page. I have custom errors off and the following in the web.config for httpErrors

<httpErrors existingResponse="Replace" errorMode="Custom"><remove statusCode="404" /><error statusCode="404" path="/404/" responseMode="ExecuteURL" /><remove statusCode="500" /><error statusCode="500" path="/500.html" responseMode="ExecuteURL" /></httpErrors>

There is a flag in the CMS which allows the /404/ CMS error page to send a 404 status code using

Response.StatusCode = 404;

This is all working perfectly well and dancy on my local Windows 10 instance, however when I deploy it to a Windows Server 2012 R2 instance and testing going to a not known URL it takes minutes for the page to load as I think it's getting in some sort of loop.

Any ideas as to the difference, I know there's some settings around the applicationHosts for SkippingIIs errors, does anyone know how to get it to work?