... if I hold the cube in my mouse pointer. :)

Slowdown when handcrafting a cube recipe

I can understand that checking every handcrafting recipe, if it includes the cube, is tedious. But handcrafting the "plastic" while running is slow-normal-slow. I just reacted to it, initially, before I "got it".

Holding it in the mouse pointer

This could/should probably be checked for, as the task to bring it from A to B, semi-automatically, will be as easy as using jetpack mod.

I'm not sure exactly what you mean. My run speed remains the same when I have the cube, whether I'm crafting with in, in the crafting queue, or holding it in the cursor. Are you seeing something different?