Performance is now on par with any other bigint library on the web, although I am fairly sure that the approach has still a lot of potential if micro-optimized.
However my end-goal is to construct rational nubers, polynumbers and matrices of polynumbers using this lib, which in itself should provide a performance improvement over just building objects/arrays with any bigint library as fields/elements.
Anyhow, if someone sees an obvious way to further improve performance, please let me know.