Confluence exports are slowed down by unbuffered reads

Confluence exports are read unbuffered. This is very inefficient and slow things down a lot, as can be seen in the flame graph in result-unbuffered.html.

The same import with buffered reads is around 3 times faster on a 5.5 GB entities.xml file, down from around 1h to around 20 min. flame graph at result-buffered.html