The UX design is not something I would judge someone on, but UX as a function of UI performance definitely is. If the UI is hitching, non-responsive, or getting into strange and incomprehensible states without giving me any indication how or why, I place the fault of that on the developer who implemented it.

It is a very good metric.

To have baseline UX awareness is crucial for every developer on any team that sets out to build a great product, just like any effective designer has to be aware of physical limitations of his design space (for example speed of light, bandwidth)

If your UX designers job is to point out that responsive UI is indeed important for your product, he is not actually doing UX design and you are likely in deep trouble.

You're assuming all products are apps or websites or something with a GUI. A product could be a library, or an OS kernel, or a web API, or any number of things, where knowledge of how to make a GUI with a good UX is completely irrelevant; or the programmer could be working on a very technical part; the engineers working on, say, Safari to optimize the javascript engine's performance similarly don't need any knowledge of what good UX means for a graphical interface.

Apple and Google shouldn't fire their best compiler/JIT writers just because they write lousy GUIs.

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.

Not necessarily, but good developers need to be thinking about their upstreams and downstreams. This is often uncomfortable because a developer personality often wants to know all the minute details and not have to worry about areas outside their core competence, but you can't build large scale software without at least a critical mass of developers thinking big picture. UX in particular depends on the task at hand, but if it's a consumer product and you're working fairly close to the surface (client or API tier) then yeah I'd expect some awareness of UX.