Improve performance

[mgirlich]

I use the paws package to work with S3, e.g. list objects in a bucket. As this took quite a lot of time I did some profiling and noticed most of the time is spend in parsing the XML response (it uses/used as_list()). I created a PR (paws-r/paws#621) that improves the performance quite a bit but is still really slow (like 90% of the time is spend in parsing). To further improve the performance without trying to use/abuse xpath further, it is probably easier to improve the performance of xml2 in general.