Performance engineering is an important activity for software architects and designers. Assessment and tuning of performance can help to make key changes in the system, especially if done early in its development. In this paper, we present a tool for the performance assessment and tuning for systems following the tiered architecture, which is a very commonly used architecture style. The web-based tool allows a software designer to specify the system under design and ascertain the different performance attributes as well as the variation in performance with load. If the predicted performance is not satisfactory, the tool helps the designer with ascertaining the changes that need to be done for achieving the desired performance. Using an iterative analysis, it presents the designer with detailed steps in terms of improvements at the software and the hardware level that are necessary to improve the system performance to the desired level. We present an overview of the analysis and tuning approach, along with an example to illustrate the use of the tool.