With Silver as Conductivity = 100,
Copper = 97.61 Yup.
Lead = 8.42 !!! No wonder car batteries get hot!!!
Tin =14.39 !!! Lead and tin are the main constituents of most soft
solders.

Ah, but the resistance across anything is:

length/(area*conductivity)

The cross sectional area of a solder joint between two copper wires
laid side by side is huge compared to the cross section of any given
point in the wire and the length is very short. Even with lead, the
joint would conduct better than the rest of the wire...

That's also why the lead car battery terminals someone else mentioned
are not a problem (large cross-section). If your battery terminals are
getting hot, it's because the contact surface between the post and the
terminal is corroded or loose.