On-premise ES in isolated network -> Delay in loading content after thingmark scan

Hi folks--

We have an on-premise experience server, which we run in an isolated environment (not on the internet, with a local router and locally assigned static IP addresses). Thingworx and the ES are running on the laptop OK, and we can scan a thingmark with a device on the same network and it will eventually load the corresponding experience.

Unfortunately, the key word here is "eventually". There is a longish delay (~10-15 sec) between scanning the thingmark when the device's "network loading" icon is spinning, and when it finally begins loading the experience content (the green bar starts moving back and forth along the top). When I publish the same experience to a standard PTC cloud ES, it loads in about 1 second from scan.

Can anyone give me a hint about what's causing this? I'm guessing it's something to do with trying to contact the GXI or otherwise trying to get to something outside of the encapsulated network environment we are using. Is there something in the ES configuration I can tweak to stop it from doing this (or at least to make the timeout smaller so it doesn't take as long to realize what it's looking for isn't there)? Thanks!

--Clay