Recording here a quick random speculation before it vanishes in the blink of an eye. We know yud (as in yud tsam zhig na “in a split-second”) and yun (as in yun ring por “for a long duration”). One may speculate that yun is derived from yud. But from what is yud derived? It is perhaps derived from yid “mind.” This is based on bTsan-lha’s recording of yid tsam, which is said to mean yud tsam. We may also bear in mind that mind is said to be like a flash of lightning, lightning fast.
Pratisaṃvid 
I would guess that both yud and yun would go back to an Old Tibetan yund.
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Oh, I checked OTDO, and see that the form yund does indeed exist!
Pt_1047 (2)
032 I ngo / gsol ba la btab na ‘prald / ⏎ du myi chad yund du gnang pa’i ngo / ‘ong ngam myi ‘ong zhes btab
149 a lug gyi ra ba rtsIgs pha’i ngo ste gzhI chIg na yund ring du ‘khod pha’i ngo / / ⏎ /::/ ryu phar thun
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