Early APR-based versions of FileAppender (1.13-1.19) called apr_file_flush() after each log request if immediateFlush was set to true. On Unix platforms, the call is a no-op on unbuffered files, however on Windows it is a call to FileFlushBuffers which can be extraordinarily expensive. So much so that the affected benchmarks were 100x slower on Windows than Linux.
I logged the issue as an APR bug (http://issues.apache.org/bugzilla/show_bug.cgi?id=33485) and have removed the call to apr_file_flush in the FileAppender. This in effectively causes the immediateFlush attribute to be ignored.