An earlier bugfix, http://bugs.xdebug.org/view.php?id=843, made sure that xdebug_sprintf always works with C locale. To do this, it calls setlocale multiple times.

One of the functions that calls into xdebug_sprintf is handle_breakpoints, which can be called many times when dealing with a large codebase. In the case of Magento 2, when preprocessing LESS files, this eventually caused a single request to call setlocale over 8 million times.