I know copper has very good conductivity, but very low resistance. It means it takes a longer time to heat properly.
Copper has good thermal conductivity, and thus low thermal resistance.
Copper also has good electrical conductivity, and thus low electrical resistance.
But can you explain why you believe either of these makes it take longer to heat properly?
First of all, objects with higher thermal conductivity will tend to heat faster, since the heat can flow more quickly throughout the part. This is the opposite of your conclusion.
Secondly, the time to heat an object depends on the thermal capacity of that object. Objects with more thermal capacity take more heat to bring them up to temperature, so they will tend to heat up more slowly. Thermal capacity is not directly related to thermal conductivity.
It turns out that for metals the thermal capacity per unit volume doesn't change all that much between one metal and another. So for an object of fixed size like a soldering tip, the amount of heat required to bring it up to temperature will be similar whether it is made of copper, or say, iron.
However, copper is preferred since the better thermal conductivity makes it better at transferring heat from the heating element to the solder joint.