Hi Dirk,
we are not aware of any performance issues for Mac OS X systems.
While you are not very specific about the performance issues you encounter, special care should be taken when using high-performances libraries such as BLAS and LAPACK (on which the DMRG code relies rather heavily). These libraries are tailored for every cpu, and you should make sure that you are using the packages intended for the cpu (s) you are using (e.g. AMD, Intel, G5, G4, ...). Apple does install these libraries with Mac OS X, and you should not need to reinstall them yourself.
Best regards, Simon