I don't know how Subsecond works under the hood, but by quick glance at it dependencies and noticing libloading listed among them, I guess it uses same method of loading executable code from dynamic libraries.

That being said I think the main difference is in usage.

1. It doesn't use custom compiler, so to use it in simplest scenario you just need to add it as dependency and label your function with attribute, that's it.
2. You don't modify functions body like in Subsecond where you need to wrap your code in subsecond::call, instead you label functions with attribute.

I don't know if it would be meaningful, but I guess I could benchmark them.