The front end is very spartanic. To watch Xmp3split work, start the program top from command line of MacOSX unix "terminal" and watch the CPU load (~25 % or, when finished, 0 %). The CPU load of 25 % leaves comfortable 75 % CPU work for other tasks.
Search words: How to split a big mp3 file / split mp3 file / große mp3-Dateien auftrennen