I also have set my firewall to prevent my 3m-50 from contacting the cloud. And I also have intermittent issues with the thermostat either failing to respond in a reasonable time (15s to 20s) or else returning in time, but with an "error" response. However, I do not believe the two are related. I think the thermostat's little web server just sometimes doesn't respond.
One other comment: I have used a packet sniffer (wireshark) and tested this:
Browse to the thermostat's "/home" page and setting its "Cloud URL:" to empty, then click Submit.
The thermostat no longer makes TCP connections to anywhere--at least not that I have noticed in wireshark.
I notice that the blank display of "Cloud URL:" does not seem to stick, it goes back to
"http://ws.radiothermostat.com/services.svc/StatIn" on its own after "a while". But wireshark is still not detecting any TCP connects. Instead of blank Cloud URL, I also tried a non-existent IP and also put the local IP of the router. All the above "worked" in the sense that wireshark shows the thermostat attempting to connect to the bogus IP and failing.
After watching the system work per factory installation for a while, I decided I am not willing to allow my thermostat to advertise to the radiothermostat website what its settings are. I am obviously an old man not of the Facebook generation. On the other hand, I do want to be able to control my thermostat from the internet side of my firewall. I have setup an openssl server on an old laptop that runs 24x7 inside my firewall such that from outside I can make an ssh connection and tunnel inside. Until someone figures out how to crack ssh, I am pretty confident that nobody else can get into my thermostat or tell anything about it by sniffing my internet packets.