Computation of performance across multiple portfolios is best accomplished by using a "cross-reference" portfolio performance report.  This will compute performance across all or some of the portfolios listed in one or more account lists.  Masking can be used to exclude specific portfolios and/or specific securities from this computation.  If you plan to run multi-portfolio performance reports, you should allow sufficient time for these computations. Such reports may require hours of computation time for very large portfolios or for large numbers (hundreds) of portfolios.