[stared]

(Another author here). It is explicitly explained in the text. :)

tl;dr: not nearly as popular (which means: less tutorials, less documentation, less examples, less integration with other systems, less community support for development or discussions)

Sure, all frameworks do have some goal and once one is confident in DL, may be a good choice. As you see from the plots there - MXNet is very fast for some applications.

[novaRom]

mxnet is not just fast for "some" applications. It consistently outperforms many frameworks especially in realm of compute intensive convolutions.

[stared]

See charts. Likely, but still 30% speed boost is not a factor for someone learning DL (then debugging, or training wrong models, can easily give overhead of 5-20x).