Sure, if you're not careful; but that's the case w/any language or layer of an application stack. I'd consider becoming familiar w/JavaScript a greater concern though. Learn to avoid JavaScript's foot guns, and use tools like Speed Tracer to help profile your JavaScript.

If it matters, my time tracking and invoicing product uses heavy client side processing, to the point where some of its reports never touch the server. We're seeing response times in the low ten's of milliseconds when relying on JS-only generated reports.