Short answer is yes. It should be able to handle many messages coming in very quickly. You’ll get a few milliseconds delay between all the readings. I have been testing with my heartbeat code with 4 endpoints and 1 gateway. The gateway publishes an event every 10 seconds to which all the endpoints respond simultaneously. There are usually a few milliseconds between each response received on the gateway. I’d be curious to test the results with 10 endpoints on the mesh.
To accommodate the possible flood of responses, you must adhere to good code design on your subscribe function. You must record the data to a variable (an array or something) and then process the results in the loop(), not the callback function. I was a bit paranoid about that so I rewrote the callback in v0.3.1 so that I did as little processing in the callback as possible.
With all that said, you might have issues doing OTA firmware updates when the mesh network is stressed. When an OTA firmware update starts, you may miss some messages published on the mesh, or the OTA update may timeout and report failure. It’s still something I hope Particle will be reviewing for future OS versions.
Edit: Here’s a typical response when all nodes respond simultaneously. Now I do realize that there may be a slight delay for each gateway heartbeat request to be received by the endpoint. That delay may be amplified when the endpoint sends the heartbeat response. But we’re probably talking microseconds.