I am sure there are ways to speed up anything but comparing Mobi7 vs Mobi8 extraction is like comparing apples to oranges. Mobi 8 format is much much more complicated as it splits then entire document into shells for each chapter and "fragments" of html , splits out css, splits out svg, etc and these need to be re-assembled into their proper order before things like internal links can be dealt with.
If the final mobi file is generated from an well designed epub, things should proceed reasonably quickly. If however you are feeding Kindlegen, a Mobi7 file as input, you will be generating a worst case situation where the entire document is one large single html file and not split into chapters and subsections as it would be inside an epub. This would slow extraction down to a great extent.
Also a 373 MB Mobi file is enormous! I can't think of any text besides a dictionary of some sort (or a graphic novel?) that would require that much space.
For more normal file sizes of under 10 MB, processing time to extract a mobi should not be significant on any late model computer.