I've never used silk touch, so that wasn't it. Maybe it was a hammer I was using...If you had silk touch on your pick that might explain it, as that reduces speed by 2. The others are correct though, bindings and sword hilts don't effect durability, damage etc; they just pass on their traits, but in the Tool Forge tools, large plates do act as components of the tool head, and traits like speed and damage are averaged between the contributing parts (eg on a hammer the mining speed is averaged between the hammer head and the two plates).