There's a UX for those things too. The users would be the consumers of the API.
Obviously, but the kind of UX discussed here is obviously that of graphical applications; nobody would say that a web API's or a compiler's UX is a "stuttering mess".
Basically everything computer that interfaces with humans is graphical. That doesn't mean it's in the designers domain to fix.
A designer is not responsible for making sure that the implementation of their design runs at stutter free at 60fps in a browser -- that is an engineers job. If the engineer who implements such a design is insensitive to these things and needs convincing that it's important to not have the implementation not stutter, jerk or jank you are never gonna get good results.
Apple and Google shouldn't fire their best compiler/JIT writers just because they write lousy GUIs.