Sablotron is a fast, GPL or MPL licensed XSLT engine fully implemented in C++. It uses the well-known Expat XML parser (included with the source code). It is available at its creator web site : http://www.gingerall.com. Okay, now you may ask : what is an XSLT engine, and what does it do? Well, it does XSL transformations i.e it transforms some XML data in a particular format into another format. Here is a tutorial on these transformations. Sablotron is not as complete as Xalan-J or XT (for example it didn't work on a docbook stylesheet), but it should do fine (and fast) for simple transformations.
Once installed, the command line for running Sablotron and applying the file.xsl stylesheet to the file.xml containing some XML data is :
sabcmd file.xsl file.xml

sabcmd is also stream-aware, so you can pipe the output of a 'cat file' into sabcmd and output the result of the transformation to screen. For example:
cat file.xml | sabcmd file.xsl

Add the '-m' flag to output the time taken by the transformation:
sabcmd -m file.xsl file.xml