Have you played this game? It often has elements from different worlds that are quickly changed. The only game I can think of that does anything like this but in slowly is the Medium.
I have the Platinum yes.
and I see not a single thing in the game that wouldn't work kn an HDD and that hasn't been done in some form before.
the portal jumps only happen during scripted setpieces, giving opportunity to hand tailor when stuff gets loaded, what gets loaded etc.
and the destroyed planet where you smack the crystals to switch back and forth are a similar mechanic as in Titanfall 2's time travel mission.
the difference is that in Ratchet the switches can only occur at predetermined places, giving the devs the opportunity to make sure the assets that need to be loaded are both fast to load and maybe even pre-loading while you approach the crystal.
and even those crystals have lile a second of fade out/in to make sure everything can be loaded in time.
Titanfall 2's level does an instant switch, which it has to because you actually have to parcour through spots while switching back and forth.
and switching in TF2 is possible at any time.
the difference between the 2 versions in TF2 is smaller than in Ratchet, I give you that, but like I said it's also instantly done and possible at any time, which is a bit mire tricky