You’re right. The form “is being” is normally used only in the case of animate beings such as humans and the higher form of animals, its sense being that of a conscious or deliberate effort by a living entity. In that sense, “My computer is being slow” would be grammatically iffy if not outright faulty. The correct sense would be conveyed by knocking off the verb “being” from that sentence: “My computer is slow.” This time, with the sentence using only the linking verb “is,” the sense conveyed is not an action but a state of being.
Of course, it’s perfectly grammatical to say “My computer being slow, I decided to buy a new one.” Without the linking verb “is,” the clause “my computer being slow” functions as a participial phrase modifier—an adjective—to convey the state of slowness.
If we want to convey the sense of a developing action, however, we can use “getting” as the main verb instead of “being,” as in this sentence: “My computer is getting slow.” The sense here is, of course, that of an action that’s just happening without a conscious or deliberate effort.
