[Retric]

The biggest issue is lack of huge caches, and the extreme latency to main memory due to the speed of light.

GPU’s are fine or even better for a wide range of compute tasks see CUDA etc, they just end up being very slow at a subset of common tasks.

[hermitdev]

Circuits don't operate at speed of light, though. People like to say this over and over again, but electron mobility through a circuit is only around 2/3s the speed of light, IIRC. They may have very tiny mass, but they still have mass.

[jabl]

No, it's the electromagnetic field which propagates at around 2/3c in the conductor. The electron drift velocity is on the order of a fraction of a mm/s.

[mycall]

It is the mass in the electrons that creates the heat.