An old subject, i know. The MLT rendering with multi threads is still taggeg "Experimental".
 unfa here :
 https://github.com/unfa/kdenlive-multirender 
suggests a proposition to multi-threads by first creating chunks of the project to export and fork every chunk to a dedicated process, and when the processes join, the final aggregation is done by ffmpeg. 
This solution seems to be very simple, but what about cutting in transition? or in keyframed effect ? Is MLT  able to handle that ?
Certainly a MLT expert can answer to these questions or if Dan Dennedy comes here ...