This article from Emery D. Berger, Benjamin G. Zorn and Kathryn S. McKinley should be read by all developers who loose their time rewriting
free. Even the worst memory allocator (the one from MS Windows ;-) ) seems not so bad comparing to custom ones.
If for some reasons they think that the default
malloc is slow they should try the Hoard Memory Allocator from Emery D. Berger which on some of my applications show a small but measurable performance improvement.