Update on increase mesh generation speed:
What I am doing now to optimize the code is to use very big objects and to identify the bottlenecks. I already discovered and eliminated 2 of them which made creating the unoptimized mesh extremely slow in a non-linear way and could easily take minutes or even an hour. Now the time needed is even less than linear (depending on object compression) and takes not more than two seconds even if the object is really, really big. Since creating the unoptimized mesh is the first step for all export and therefore always required, this is very good news.
Export using vertex color and without optimization is lightning fast now.
Unoptimized export using a regular texture still is very slow because the uv layout optimization takes forever due to the high quad count. This is easy to solve but makes me wonder: why would anyone use this combination (no optimization + texture) anyhow? If you use edge bleed then the texture will become very big (at least 3x3 pixels per quad required instead of one), if not, then render results are very bad.
If not using optimization, only vertex color and simpleTex are reasonable options. On the other hand, if optimization is wanted then simpleTex can't be used. Long story short: I will remove simpleTex from the material option and use it automatically if no optimization is selected. This not only removes an unreasonable way to export but automatically removes another bottleneck.