1.Xincluder should have option incremental="true/false".
2.If incremental="true" output file should be changed only if source file was changed.
3.It is not problem, that incremental="true" will get more time than incremental="false". Objective is "to do not touch target file". Ie target file is source for a lot of XSLTs and xcluder changes source file for XSLT every build.
Target file of xcluder shouldn't be replaced if intermediate file with output is exactly identical.