I'm not sure how that's any better than a splash screen in fact I think it could cause much confusion.

I created a web app recently that had to make an Ajax call but could not allow any user input until this action had completed as various parts of the UI would have to be refreshed with new information.

I had reports from users that they thought the app had crashed since it wasn't responding to their inputs for a few seconds. It turned out the best solution was to simply whiteout the entire page and show an animated egg timer until the action had completed.

It's only really heavy professional apps that take much time to start anyway.