Performance tests mimicking real usage

[note]

There are already some simple tests but they're very synthetic. They're useful in the sense that they allow us to easily find what the bottleneck is. Besides of them we should have tests mimicking real world usage (doing some transformations on real world XML, trying to operate on quite big files (e.g. a few MBs may be also interesting).

Would be nice to add test results to doc (probably a separated MD file not to clutter the main docs)