I am sure they are. Try to put a lot of assets to the second layout and try to switch between them with System -> go to layout. You will notice that the first time you go to layout 2 takes time, but already second time is very fast (at least on the devices which have enough RAM to hold the data).
I think it's pretty good practice to put the assets on the first layout because it's the only one layout where you can implement the progressbar, so player at least see that something is being loaded.
But! It obviously depends on the project size. I think it's a good practice only in a medium size projects. For small projects there is no point since there are usually few small assets. ,
Now in big projects you might have really a lot of stuff which would be really a pain for user to wait for all of them being loaded. In such case you should have it organized on the layouts that use them.