I'd never heard of the heat pump dryers until this thread. Every day's a school day.
The are notorious for taking forever to dry anything.
I thought that was condensing tumble dryers.
Heat pump dryers are a type of condensing dryers. The heat is pumped from the condenser to the heater. The air goes over the condenser where the moisture, unsurprisingly, consdenses. The air is then heated by the heater and goes into the drum where it heats the clothes and picks up water from them. Then it's back to the condenser. Getting the heat from the condensing of the water is the best source of heat to pump.
Heat pumps are a significant cost to build, and are why heat pump dryers are more expensive. Making them more powerful would add to the cost. Keeping the power down will probably make them more efficient, but will slow down the drying. On a vented dryer, a more powerful heater will be hardly any more cost than a less powerful one. Also, it may use less energy to dry the clothes faster, as less air will need to be blow through the dryer and heated if the process is takes longer.
Our dryer says it will take 3 hours when set to "cupboard dry" but will take much less if there isn't much in it. It's probably slower than our previous vented one. It really doesn't cause a problem for household use.
One other advantage of heat pump dryers is that they are far less likely to cause any fluff to burn. If fluff lands on an electric heater, that part of the heater will still generate the same amount of heat, so it will run hotter. On a heat pump dryer, the heat comes from the condensation of the refrigerant gas, so if one area of the heater is covered if fluff, there will just be less condensation of refrigerant in that area so less heat will be generated in that area and the temperature won't go up much.