I have not had any experience with anything I'd call a "deliberate speed slowdown." I suspect this is yet another Internet Legend.
I rarely have downtimes due to weather unless it's a sever thunderstorm right in the line of sight.
I used to have "enhanced ADSL" (meaning I had fixed IPs) and NEVER remembered experiencing ANY downtime, but I heard from others who had the DHCP-type ADSL, and they said they were down about once a week, sometimes for a day.
Cable modem, I have no experience with such downtime, so I can't compare.
So how much 'downloading' do you typically do? multi-MB product downloads? 100 kb pdf files?, large jpegs?, etc?