Electrical solderless or soldered connectors

Mattax,
Solder may well have higher resistance to copper. While you might have a 3ft long battery cable to your starter, you would not use a 3ft long length of solder for the same job.
The resistance of any material is a product of it's length & CSA. The distance between a copper wire inside a terminal & the distance to the wall of the terminal [ the space that is filled with solder ] is so small that it would probably be measured in microns.....The resistance across the joint would thus be so tiny that laboratory grade equipment would be needed to measure it.
More solder heaped on the joint will further lower the resistance because the CSA is being increased.