Mind you, I was just looking at a roll of 'speaker cable' on offer in a DIY store, and I reckon spider silk would have a greater cross section.
Recently, I bough 100 m of two core 0,75 mm^2 cable.
A problem with it came to my attention after I cut off a decent chunk for a friend and a few days later he complained that the cable is crap and that he can't solder it. I took a piece, tried tinning it and it went well, no issues, so I talked to the guy and asked him what was the problem. After all, he's 72 and has been tinkering with electronics for a good part of his life, so apparently there was a problem. He said that the cable takes solder once, but never again. I asked him to repeat that.
So, I take the cable again, tin it (no problem) and try to solder two ends together, but nope, it doesn't work. The cable repels the solder and I was unable to make the joint. I've never seen such behavior before.
I tried the flame test, and sure enough, it's copper clad aluminium.
I bought it in an electronics store. I asked for 0,75 mm^2 speaker cable, so yeah, if somebody bought that crap expecting copper, I wouldn't be surprised if they didn't get the result one would expect. Or if they are used to that, a proper quality copper cable could make a difference.