Maybe something has changed since last year - I haven't tested it recently. However last year when I was releasing my trivia game I clearly noticed that every layout first loading took on my mobile approx 2s. Then every layout second (and n-th) loading took about 0.5s. So the difference was clerly noticable.
Now regarding RAM. It depends on how the program is written. I am not sure how C2 works atm. But system does not clear any RAM data "by default". Clearing RAM takes time as well so data usually stays there even if it is not used and then some GC (Garbage Collector) clears it periodically. Or you can do it manually by coding as well ofc.
And let's say I have an app exported to desktop. My laptop has 16GB of RAM. It is "strong enough" to keep in RAM whole game even if it is not being used. But access to the RAM memory is much faster than HDD read so the game would be much more optimised if all the data would remain in RAM rather then be cleared and loaded everytime the layout changes.
For mobile devices it is a bit different story cause we usually have 1-3GB of RAM which is not that much and indeed sometimes RAM might be not big enough. In such cases unused data will be overriden by the new data.
I am not an expert, just saying how I see it based on my knowledge and experience.
Now the last thing to say here is that putting all objects in one layout is not a good practice when you think from the perspective of being a good coder. Good code is semantic and hermetised code, which means (in short) that every "thing" should be where it belongs and nowhere else.
But loading everything on first layout is acceptable even for good developers in C2 simply because of performace.
But then again if for some reason data is cleared when you swich to other layout... then there is absolutely no point doing so. It requires testing on various platforms and various mobiles to be sure how it works I believe. Nice topic tho!