A typical electric American-dryer uses about 2.5kwh for a large load of clothes, according to Consumer Reports magazine.
If we were to look past the additional 600w motor's power-draw over a longer period of time, and had sensors to stop the machine when the load was (at the same level of) dry, I'd say the energy use either way (high or low heat) is so close that it does not have a truly measurable impact on one's monthly bill.
High heat does fade darks and potentially leads to shrinking. (But then again when I was 10kg/25 lbs. lighter in weight, was a "perfect" (shrunken) medium in cotton tops/shirts. LOL
SO for me shrinking was needed and expected.
Wasn't there some feeble attempt to run/sell a dryer for 12 hours (overnight) at (or just above) room temperature to get it an "A" energy rating? Was that the UK or the European Union?
My full-sized electric dryer, by necessity, runs its heating element on 110v (no 220v available or existing in my apartment) and thusly heats at 1,250w instead of 5,000w (@ 1/2 rated voltage, 1/4 the wattage). When I multiplied it out based on the extended drying times, the total KWH used was still very close to 2.5KWH per load.
In many markets in the Untied States a gas dryer can cost only 1/3 of what it cost to run an electric one. (Barring hydro-electric markets and parts of the south first getting gas and paying royally for that infrastructure).