Distributors entering Flatpakland

I think you mean snaps. Flatpaks don’t mount squashfs over squashfs (although the recent Ubuntu and Firefox snap kerfuffle wasn’t purely about filesystem speed; I think someone had unpacked Firefox’s snap into a directory, launched it, and it was still slow).

But you do end up with multiple shared libraries from different runtimes keeping being loaded again and again, so startup time won’t be negligible anymore.

And yeah, this is my beef too. You work hard so with PIC binaries all over the place you don’t need to play with relocations anymore, now filesystem cache and mmap() can work about as well as it gets, and then you ruin it by making all executables bring their own slightly different versions of shared libraries. There have been times that RAM and storage and electricity seemed cheap and getting even cheaper; sorry, looks like these times are over.