SharePoint REST api call times out

We are using Nintex Workflows to make web requests to the _api/contextinfo address, which mostly responds with a time out error. After 4 or 5 attempts it may respond (and I am able to extract the digest token).
We have checked the ULS logs and it seems that SharePoint always responds within 30ms.

We have tested using PowerShell, but SharePoint always responds within 30ms.

It looks like the issue in not with SharePoint or Nintex but somewhere in between.

Any similar experiences?

Any suggestions where to look?

thanks.