MQTT publish rate of Photon

[ScruffR]

That would greatly depend on the MQTT library which was contributed and isn't Particle's code.
The underlying TCP layer is definetly fast enough for several KB/s

Look here

Sending large data off Photon using TCPClient

I was wondering exactly how fast you can get data off a Photon using a direct TCP connection (TCPClient), and how reliable is it? Well, after tweaking the code and some parameters, in my somewhat arbitrary test of opening a TCP connection and sending 1 Mbyte of data up to a local server, then closing the connection, I got an average of 805 Kbytes/sec. 1.3 seconds to upload 1 MB of data. I repeated this test continuously, uploading almost 4 GB of data in 86 minutes, and there were no failures. …