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I would expect that to all happen quickly - ready() is called as soon as the object is ready to process, then _process(delta) is called every frame so the next line will appear 1 frame later. The the third line will print after you count to 10 milliion (which does not take long - on my computer that is 490ms)
The approach you are taking will be very inefficient - you are asking your CPU to count to 10 million as quickly as it can. That will max out your CPU until it is complete. The length of the delay will vary depending on how fast your computer is, so that will not give you a predictable delay. Also, when you run that inside _process you are asking Godot to do that every frame - maybe 30 or 60 times a second. Forcing the CPU to 100% is not good practice :-)
Much better to use a Timer. Documentation is here. The principle of the Timer is that you setup the timer, then it runs for a configurable time (using minimal CPU) and then calls a configurable function when it is done. Check out the docs for details.
You can add timers in the scene tree. There is a nice tutorial here or you can find many more online
Or, a shortcut to add an inline timer is:
This line pauses your code for 3 seconds and lets Godot do something else. When the timer is done, it comes back to where it was.