Performance is usually the last thing anyone designing a website/app thinks about, after all, the usual response is to just throw more memory or processing power at the problem and be done with it. You may have one of the best designed sites but if your users experience any lag times they may go elsewhere causing drops in your revenue stream. Nipping performance issues before they happen or following the 6 P's (proper planning prevents piss-poor performance) is the optimal solution.
A progress bar is not the solution.
Key to Glass responsiveness is.