How about an option of convert plain txt to "plain" mobi, no sections, no fonts, just like read a plain txt file, but in mobi format.
I tried to create the html from a 10M txt file with a simple python script. Inside the <body> tag, there are only <p>..<p>, or only <br>, then feed that html to ebook-convert, it still runs not fast enough.
I wish I could contribute, but the task looks daunting for my skill level...