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 perceived throughput. Just my thinking. Scott Joe Greco wrote:
http://www.wired.com/gadgets/miscellaneous/magazine/17-10/ts_burningquestion
It used to be that we would notice this, except that it had everything to do with temperature *and* dampness. In the '90's, it was still quite common for a lot of older outside plant to be really only "voice grade" and it wasn't unusual for copper to run all the way back to the CO, through a variety of taps and splice points. Even though Ma Bell would typically do a careful job handling their copper, the sheer number of potential points of failure meant that it wasn't unusual for water to infiltrate and penetrate. If I recall correctly, the worst was usually a long, hard cold rain (hey we're in Wisconsin) after which people who had been getting solidly high speed modem connects would see a somewhat slower speed.
... JG