WEBARM enables end-to-end response time measurement for Web applications through Web page instrumentation. WEBARM-agents are a form of mobile code capable of moving with the Web application to monitor response time. WEBARM includes the software components responsible for interception and processing of the WEBARM API calls including support for response time measurement, collection, and communication. This work provides an assessment of WEBARM instrumentation impact on application response times as well as insights into the design issues involved. We first provide a general overview of alternative approaches to Web response time measurement. We then consider alternative WEBARM agent software designs, focusing on techniques to store timers across Web page references and communicate logged data to a server. Next, we present measurement results for the implementation alternatives to provide insight into the overhead involved with instrumentation and the developed designs. Finally, we present conclusions and a description of future work