I can't agree with that. I checked Photoshop recently. It queues every user action. And it respects user and do not throw any his actions away.

o I tried to paint about five gradients very fast. You can paint it easily. While the first one is calculating, you can draw the "tool outline" of the second gradient, release the mouse and it'll be added to the queue. Then you just need to wait about 0.5 sec until the queue is processed.

o Test on mixing different types of actions. This works as well. I started image type conversion (rgb->lab) and painted with the brush. I was informed by the program that i wouldn't have any visual feedback with a "Busy" cursor. After the conversion has been finished, it replayed my stroke as I painted it. I think this is really wise and logical decision made by PS team. Let the user decide what he wants, to paint or not to paint.

Btw, the latter test is very important, because it make the workflow much more smooth.

A usecase: you can start a conversion, then press e.g. Ctrl+M hotkey. It will open you a Curves dialog *after* the conversion is finished. So you'll have about 2-3 spare seconds to think or to have a rest.