It is interesting that this question has been posed. How were you planning to describe such performance information anyway? The challenge in this area is that there is no WS-Policy based standard for asserting performance or SLA information yet. However, I would expect that the standards will evolve to address this issue.
For now, I would advise to keep track of meta-data, including performance and SLA information in a SOA registry. Registries today allow you to specify information such as execution time and service availability metrics, and allow to be extended for more. The specific metrics you select depend on what your consumers are looking for, but it is important to find a way to maintain and update such performance metrics so they don't become outdated. Your Web services management (when the service is in production) or testing products (while it is being evolved before production) can feed the latest performance data into the registry, otherwise such information would become unreliable if it is left for manual updates.
This was first published in June 2008