Natiu-mqtt has matured quite a lot since a couple months ago. It now has a Client implementation which is concurrent safe and extremely fast and small- It uses 100kB of static memory on a microcontroller and just 5kB of RAM; compared to 212kB static memory used by Paho.
Full Changelog: https://github.com/soypat/natiu-mqtt/compare/v0.4.0..v0.5.1