tbb brass axles are thinnest - about 5mm. tmbr and no jive axles are about 6-7mm (the same thickness of most c-bearing yo-yo’s bearing seats). proyo2 and spintastics axles are about 8mm, and most “wooden kk’s” come in close to 10mm at the thinnest. that’s a lot more wood contacting the string. the thinner those inserts get at the center concave, the more fragile they become and the more the walls of the concave touch the string, slowing rotation.
https://farm6.staticflickr.com/5575/14805237199_fa7296bfc6_z.jpg