This should have been my first comment, but there you go.
Hello, I'm Roger, the Mosquitto project lead. Thanks for the kind comments, I'm happy to answer questions!
I work for cedalo.com developing Mosquitto and other MQTT projects. We offer Mosquitto based support, please get in touch if that's of interest to you.
I can't comment on MCUs or WiFi BSSIDs, but on the server side I have an Intel Atom N2800 (think low performance) with nearly 4000 clients sending out ~50k messages/s with an average payload size of 80 bytes. The overwhelming majority of those are not using TLS. About a quarter using MQTT over websockets.
That sounds really nice, I love interesting projects in areas outside of my experience. If you're using Mosquitto I'd love to hear more about it - my contact details are at the bottom of https://github.com/eclipse/mosquitto/ :)
As others have pointed out, that's not the case. Something you do have to be careful of is the compile options for libwebsockets and what version you're using. Recent-ish version of libwebsockets in particular disable by default some features Mosquitto relies on, so some installations may suffer.
Version 2.1 of Mosquitto will no longer need libwebsockets (although it can still be selected at compile time) and so websockets support will be less dependant on how other libraries are compiled.
The lowest spec device I've come across Mosquitto being used is a 180MHz MIPS processor on 32MB RAM, running Linux.
Someone has recently taken the RSMB broker (from IBM originally) and ported it to the ESP32: https://github.com/DynamicDevices/picobroker I can't comment on what the limitations of that are though.
Like your other reply said, MQTT-SN can use UDP so may be a better fit in some situations. It's a shame that MQTT-SN hasn't seen wider use, but there isn't that much support at the moment. The protocol is being standardised at the moment, and there has been an MQTT to MQTT-SN gateway as part of the Eclipse Paho project for a while now. The Mosquitto project has the aim of adding support for MQTT-SN - it's just not as high a priority as other things at the moment unfortunately though. If it's something that's important to you, please get in touch.
Hello, I'm Roger, the Mosquitto project lead. Thanks for the kind comments, I'm happy to answer questions!
I work for cedalo.com developing Mosquitto and other MQTT projects. We offer Mosquitto based support, please get in touch if that's of interest to you.