Why does PETG require slower speed?
The density of PLA is around 1.25 g/cm³ and the density of PETG is around 1.38 g/cm³. When you're talking about the amount of energy needed to melt a particular volume (which is what your extrusion units are) rather than mass, you need to scale the heat capacities (with units of $\frac{\mathrm J}{\mathrm g\cdot \mathrm K}$) by the density to get $\frac{\mathrm J}{\mathrm{cm}^3\cdot \mathrm K}$. This brings their volumetric heat capacities somewhat closer: 2.25-2.63 vs 1.52-1.79 (about 47% higher for PLA rather than your figure of about 62%), but with PLA still higher.

You also have to account for heat loss to the environment. PLA is typically printed around 200°C or 210°C at most; PETG in my experience requires 250°C to reach low enough viscosity to be printable at any speed. Assuming an ambient 20°C, the rate of heat loss should be something like 25% higher for PETG. So the hotend has that much additional energy needed to begin with.

Beside that, PLA is printed at temperatures where it's still extrudable and able to bond even if the temperature drops significantly below the nominal nozzle temperature (down to 180°C, maybe even slightly lower), whereas PETG has trouble with increased viscosity and poor bonding right away if temperature drops.

Going broader still, PETG seems to need to keep its heat longer after being extruded in order for layers to bond well. (As evidenced by the need to lower fan or turn it off completely.) A slow-moving nozzle both provides heat (from the proximity of the nozzle itself) to slow the cooling, and reduces air flow across the part (by not causing as much air flow itself just by moving).