If thick axons are faster, why are most axons in the brain thin and slow? · Grey Matter

Speed grows slowly with width while cost grows fast.

If thick axons are faster, why are most axons in the brain thin and slow?

Speed grows slowly with width while cost grows fast. In a bare axon conduction velocity rises roughly with the square root of the diameter, but the volume the axon takes up rises with the square of it, so doubling the speed means quadrupling the diameter and making the cable sixteen times bulkier. Wider axons also have more membrane to recharge, which costs pump energy with every spike. The brain therefore keeps most local wiring thin and slow (a millimetre takes about a millisecond even at 1 m/s) and spends myelin and width on the long paths where delay matters.