Trying to do fast power chord downstrokes

I'm trying to do the fast downstroke power chord section in Deep Indigo, specifically the part that begins at 0:54 in this video: https://youtu.be/JYvDOIAhh9U?si=Dyx6gUmem47olXdm&t=54 Attached is a video of me attempting this, which is pretty poor. Not only is it sloppy and not exactly on tempo, it's very tiring. I've watched some videos from the Obsidian seminar which explain this a bit (e.g., https://troygrady.com/seminars/obsidian/chapter-1-downstroke-mechanics/) , but I'm still struggling: as recommended in the video I linked, I'm using a downwards slant, and I believe my motion is similar to the one used by the instructor. But especially the movement where I'm going back to the top to do another downstroke feels very unnatural and "heavy".