How much of the speed-up is attributed to not hardening the code?

And i do not mean this in a flippant way, as how to harden with speed in mind might alter how to design the format and the codec.

Almost none. Once again, simplicity comes to our rescue here. The decompressor is simple and a naive safe version I implemented but haven't merged into main yet (see: https://encode.su/threads/4514-misa77-ridiculously-fast-deco...) is only ~5% slower than the current unsafe version (and can very likely be made faster).