Are geo movement speed feet broken? They feel slow.
Nah, they're fine. I dual box and often /follow my main with my alt's GEO wearing Geomancy Sandals +3. When I'm wearing 18% movement speed on the main, the GEO doesn't fall behind. In comparison, when I'm using something like WHM BLM with 12% feet on the alt, eventually it will fall further behind if the other character is using 18%.
Though if you feel like you're getting no movement+, make sure your lua is correctly swapping the right piece in?
EDIT: Oh I see you turned it off and still seem to be having issues. Not sure what it is, but it's not a flaw with the shoes themselves. Those work just fine for me, and I haven't seen others reporting an issue with them.
Another thoughts: make sure you aren't using something on your other character that makes that one move FASTER than usual (no judgment lol), maybe you have something like that set up inadvertently and it makes the "correct" speed on your GEO Taru feel slower based on the comparison?