I may be having my wires a little crossed (I'm not an electrical
engineer) but I was always under the impression that manipulation of the
physical characteristics like that from heat/dampness didn't reduce the
"speed" but the "quality" (like line noise/errors/etc) of the line.
Whether old telco lines or newer data lines it's all about electrical
signal and bit error rates. More errors = more retransmissions = slower
Just my thinking.
Reduced quality results in reduced speed. In the best case, you have a
technology like DSL that detects the reduced quality, and dynamically
adjusts transmission characteristics to adapt. In other cases, you have
a technology like Ethernet where misdetection of bits results in the
loss of a packet, and ultimately requires retransmission.