Hello,
I have built a basic Instant App in ArcGIS Enterprise and it is very slow to load in both Chrome and Edge. There is a stage where it pauses for exactly one minute every single time, then once it has loaded, the speed is fine. This happens on every Instant App I build in AGE (ArcGIS Online is fine). We have an 11.2 and an 11.3 environment and it happens in both.
I used the browser console to investigate the issue. The app makes this request: https://maptestportal.cornwall.gov.uk/portal/sharing/rest/portals/self?f=json, waits for a minute for a response and then the request is cancelled:

If I open that URL in a new browser tab, it responds within a few seconds.
I have made an app on our test environment with a simple web map and shared it to demonstrate the issue: maptestportal.cornwall.gov.uk/portal/apps/instant/basic/index.html?appid=145c07cdabbc49f2879c104c5d37f40f.
We have previously logged a call with ESRI UK and they think it is an issue on our side, but our Network team have been unable to spot anything. Any help would be gratefully received.
Thanks,
Sarah