I think picking FCPX as an example for a low-cost laptop is not a good example. We should be comparing every day programs used by regular users, such as Mail, Safari, Messages, iTunes, Facetime, etc. They would probably be the exact same programs we're using today but "fat compiled" for both macOS x86 + ARM instead, kind of like the programs in the PowerPC-to-Intel transition era.
There's also "legacy apps" to take into account. If Apple does release an ARM-powered low-cost MBA replacement, it will need to have a "Rosetta 2.0" to be able to run x86 programs. And then speed becomes a matter of how much time is spent calling APIs which would be native code for macOS ARM and how much time is spent emulating x86 program logic which is where the slowdowns would occur.
Then again, we don't know how many ARM cores the laptop would have, maybe a quad-core ARM can emulate a low-end dual-core x86 intel processor in real time? And since Apple can put quad-core ARM processors in smaller things like an iPad, maybe this ARM-powered laptop could have more than four cores.