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PLA has a heat capacity of 1.8-2.1 J/g-K, while PETG 1.1-1.3 J/g-K. This means that each gram of PLA needs more energy to heat up. I assume no "melting latent energy", since we talk about plastics.
The density is about the same.
Still, printing speed for PETG is said to be kept at max at 60 mm/s, while PLA can easily go up to 100 mm/s.
Why is PETG supposed to be printed slower than PLA?
Edit: a link to a more recent question may be of interest: Power consumption of filament extrusion
I'm adding this answer to somewhat challenge the findings of my original answer, and the premise of the question: PETG does not need lower print speeds, and can even be printed at higher speeds than PLA under some conditions due to reduced need for cooling. You can see this from some of the "#speedboatrace" entries printed with PETG. So what was really going on with the original claim and my agreement with it?
I think my original answer is still somewhat true: it's likely that it takes more hotend power to melt PETG at a rate that can be successfully extruded and bonded than to do the same for PLA. But there are other factors at play in the perception that "PETG has to be printed slow".
FarO did not specify details of the printer(s) in question, but I found the big limiting factor for my Ender 3 printing PETG was the stock extruder, which presumably was skipping bad to begin with, and even worse with Linear Advance, trying to keep the filament under high pressure to compensate for its compressibility. Since replacing the extruder with a direct drive one, I've had no problem printing PETG at the same speed as PLA, and both can print much faster than I ever could with the stock bowden extruder.
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).
