XML parsing performance

[ynfle]

`-d:danger` takes away bound checks and other safety mechanisms see 
<https://nim-lang.org/docs/nimc.html#additional-compilation-switches>. You 
could use any of``-d:release -opt:speed -d:lto``. Also try changing the garbage 
collector to `--gc:arc` (though I'd suggest using the latest version of nim, 
which 1.4.6, as there are many bug fixes along the way)