Author here, I'll check this every so often and try to answer any questions…
One question: compared to the native text-to-speech on macOS, the synthesized speech sounds, for a lack of a better word, robotic. Is this an inherent property of the approach you used or a result of trying to squeeze something as complex as this into a Javascript library?
However, all the configuration data, including phoneme tables, may be overwritten (but you would have to install eSpeak on your machine first, in order to compile these.)
Another approach would be actually porting this to JS (instead of cross-compiling), by this having full access. But I simply do not have the resources for this. (Meanwhile, there's the Web Speech Synthesis API. With this being available on most modern clients, it's probably not worth the effort.)
Mind that the core won't run concurrently as a worker on mobile devices, but rather as an instance in the main/UI thread. This is, because mobile devices will mute the playback triggered by a message from a worker, as there is no immediate user interaction. Therefore, longer utterances are likely to block the UI noticeably, while the internal sound file is processed. This is a bit sad, but how things are.