I have seen the same issue with my devices connecting with 3G, LTE Cat3, Cat1 and CatM1. The devices maintain a persistent TCP connection and messages from the device instantaneously arrive at our server, and messages sent using the Hologram API also quickly arrive at the device.
After a SpaceBridge tunnel connection is established to the device (a terminal SSH or Telnet connection), if I’m interacting with the device I get a reasonable response, but if I let the session idle for 30 seconds or so, then it takes 5 to 10 seconds before I get a response from the device, and then it’s quick until I pause again. I have seen this with both AT&T and T-Mobile.
If I use these devices on MVNOs that provide a VPN to a static IP, I don’t see this issue, so I think it’s something unique to the Hologram Spacebridge implementation.