Hi and welcome!
It definitely looks like the motion itself changes at the slower speed. There’s still elbow, but the instability in your hand allow other things to move (either wrist flex/ext and possibly even forearm rotation) and I think I might even see the grip itself move a little.
Some things you could try:
- Don’t tense/bend your index finger so much
I am pretty flexible and I can’t recreate that grip without some discomfort. Maybe try a more traditional trigger or ‘relaxed’ trigger grip like Rusty does:
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Try not using your other fingers to graze the body of the guitar. I used to do this too, particularly after getting into Batio’s playing. I found when you go all in on an elbow technique sometimes it can cause things to get a little ‘sticky’. And overall, ‘sticky’ is the problem as we see the pick just going nowhere at the slower speeds. You’ll of course need to feel some degree of attachment or ‘anchor’ to the instrument, but you can get that in an elbow motion with a different tactile reference - the thumb’s palm heel. Notice how much Rusty contacts the strings in that area. Which brings me to my next suggestion…
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Try to pronate just a little more
More broadly, I agree that at various speeds the motions are hard to do. There are pockets of tempos with various motions that I just can’t do. It’s sort of like asking someone to jog the same way they sprint.
- You are a paid member, so take advantage of the Technique Critique feature on Troy’s main (i.e. non-forum) site. Once you’re logged in, go here and it will walk you through the steps
That gets you a guaranteed response from an actual expert, not a wanna-be shredder (though very serious hobbyist) like me 
Anyway, best of luck, you’ve come to the right place! You may also want to post of video of yourself playing lefty just so we can see the differences.
