Hope this might help someone else.
It did Mark. It is brilliant. Thank you (and @Rpgvoyage) for posting your findings and the iOS Shortcut.
While searching the forums I couldn’t find anyone complaining about the performance issue while editing. Quite the opposite, there are numerous examples and tutorials showing highly complex videos showing smooth scrubbing and flawless previewing, while I was struggling to put together a single timeline with just some simple transitions (on an iPad Pro 2018 12,9”). On a three minute video, just about halfway, the preview would become erratic, skipping transitions and bits of some clips. True, the final rendering would be flawless but trying to synchronize transitions with music while editing was nearly impossible and very exasperating, which is sort of the opposite of what editing should be.
I was convinced that I was doing something wrong and about to email LumaFusion’s support team when I stumbled upon your post and decided to try it out. I created two identical projects, one using the .mov files imported from the Osmo Pocket (4K 60fps) and the other one using the exact same clips transcoded to H.265. I added the same generous amount of transitions to both. Sure enough and even with the preview set to “fastest” the first project soon started to crumble while the second one, even after setting it to “best” and adding a gazillion of extra random effects, kept running smooth as the proverbial baby’s bottom.
Once again many thanks for taking the time to share your findings. Video editing is fun again for me.
Ps. Just an idea but maybe including this workaround as a “tip” in the Reference Manual or including it on a future tutorial would save a lot of frustration to some LumaFusion users.