from the comments:
These skills, tools, and best practices are specific enough that there is a niche industry forming around it. Check out the O'Reilly Velocity conference for justification that there is enough demand for this type of community.
I agree that there might be a niche industry forming around web performance. But I'm pretty sure web performance questions would of interest to many outside that niche industry, mostly programming related, easily findable through the performance tag, and get high quality fast answers on SO. I don't think we'll get that much benefit from the fragmentation.
I think it should certainly cover back-end performance issues as well.
This would probably be on-topic for serverfault. Webmasters should cover anything else.