I doubt that, but have actually no idea. But for sure, there is some tiny difference in which filesystem you format your card to and if memory serves, in what setting you use (are blocks still a thing?). And what’s most efficient for you depends on what size files you are transferring. Someone more familiar with i/o operations might have better thoughts on this.
Regarding the speed difference, those who have tried multiboot using SD card: how was the experience regarding responsiveness vs. eMMC? SD would give more space but is the downside too inconvenient.
At one point there was some talk of trying swapping memory to modem or wifi card slot (probably needs an adapter to physically fit it). Or do I remember wrong? I don’t think anyone did. A curiosity in stead of a viable option with L5.
The Flatpak bloat - cleaning out? thread had a good link and the Low disk space - how to clean out garbage? - #13 by joao.azevedo had some good notes. But this important info is almost lost to the forum. It’s not for instance offered via an app (how about in mobile settings or shop the info : “this is how much your flatpaks take, here are the alternatives and this switch lets you clean not needed”, as a way to keep device in good condition?). Which leads my runaway train of thought to: is this partly a problem of the (un)curated software shop that is not serving the users efficiently?