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A methodology is presented to compute performance measures for distributed information systems with unbalanced flows (i.e., number of transaction leaving a server is not the same as number of transactions entering that server) due to asynchronously spawned parallel tasks--an increasingly important phenomenon in modern information systems which has a significant effect on performance that cannot be analyzed by classical queueing network models. A decomposition method is applied to decompose the unbalanced flows. Formulaw for open queueing networks with unbalanced flows due to asynchronously spawned tasks are developed. Furthermore, An algorithm based on Buzen's convolution algorithm is developed to test the necessary and sufficient condition for closed system stability as well as to compute performance measures. An average of less than four iterations is necessary for convergence with this algorithm. A study of the INFOPLEX data storage hierarchy has been conducted using this rapid solution algorithm and detailed simulations; highly consistent results were obtained. A cost effective software tool, using this methodology, has been developed to analyze an architectural design and to produce measures such as throughput, utilization, and response time so that the potential performance problems can be identified. (Author)